Set S is given as S = {1,3,5,7,9,11,13,15,17}. In how many ways can three numbers be chosen from Set S such that the sum of those three numbers is 18?
Which of the following statements must be true?
I. The product of first 100 prime numbers is even
II. The sum of first 100 prime numbers is odd.
III. The sum of first five nonnegative even numbers is divisible by both 4 and 5
Correct Answer : e
Explanation: (a) The product of first 100 prime numbers will always be an even number , because it will contain 2 as a factor , since 2 is also a prime number
(b) 24,133 is the sum of the first 100 primes. Hence it is an odd number, so the statement is true
(c)The number series 2, 4, 6, 8, 10. Therefore, 40 is the sum of first 5 non negative even numbers. Hence it is divisible by 4 and 5
If x, y, z and w are positive integers, is x odd?
(1) 7x + 8y + 4z + 5w is odd
(2) 3x + 2y + 8z + 2w is even
If x and y are positive integers and x is odd, is xy even?
(1) x^{3}y = 6a^{3} + 23 where a is a positive integer
(2) x^{2}+y = 3k + 7 where k is a positive integer
If n is an integer, then which of the following statements is/are FALSE?
I. n^{3} – n is always even.
II. 8n^{3} +12n^{2} +6n +1 is always even.
III. √ (4n^{2} – 4n +1) is always odd.
If x and y are integers, is 3x^{4} + 4y even?
(1) x^{3} is even
(2) y^{2x} + 3 is even
Steps 1 & 2: Understand Question and Draw Inferences
We need to find if 3x^{4} + 4y is even.
4y will always be even, irrespective of the value of y, because an even number (4 here) multiplied by a number (whether even or odd) always gives an even product
> For the sum of 3x^{4} and 4y to be even, 3x^{4} must be even. This is because, the only way sum of two numbers can be even is if they are both even or both odd. Here, we have already determined that 4y is even.
3x^{4} is the product of an odd number (3) with x^{4}
This product can be even only if x^{4} is even
x^{4 }will be even if x is even
Therefore, in order to determine if 3x^{4} + 4y is even, we only need to find if x is even.
If x is even, 3x^{4} + 4y is even.
If x is odd, 3x^{4} + 4y is odd.
Step 3: Analyze Statement 1
x^{3} is even
> x is even
Sufficient
Step 4: Analyze Statement 2
y^{2x} + 3 is even
> y^{2x} is odd ( Odd + Odd = Even)
Now, an odd number multiplied with itself any number of times will give an odd product (similar result holds true for even numbers as well)
So, if y raised to the power something (2x in this case) is odd
> y is odd
We do not get to know if x is even or not.
Not Sufficient.
Step 5: Analyze Both Statements Together (if needed)
We get a unique answer in step 3, so this step is not required
Answer: Option (A)
If X is a positive integer, is X^{2 }+ 1 an odd number?
(1) X is the smallest integer that is divisible by all integers from 11 to 15, inclusive.
(2) 3^{X} is an odd number.
Steps 1 & 2: Understand Question and Draw Inferences
For X^{2} + 1 to be odd
> X^{2} must be even
> X must be even
We need to find if X is even or not
Step 3: Analyze Statement 1
X is the smallest integer that is divisible by all integers from 11 to 15, inclusive
> X is divisible by 12 and 14
> X is even
Sufficient.
Step 4: Analyze Statement 2
3^{X} is an odd number
An odd number multiplied with itself any number of times will give an odd product
> 3^{X} will always be odd, whether X is even or odd
Not Sufficient.
Step 5: Analyze Both Statements Together (if needed)
We get a unique answer in step 3, so this step is not required
Answer: Option (A)
If a, b, and k are positive integers, is the sum (a + b) an even number or an odd number?
(1) a = ( k^{3} + 3k^{2} + 3k + 6)
(2) b = (k^{2} + 4a +5)
Steps 1 & 2: Understand Question and Draw Inferences
We are given that a, b, and k are positive integers. And, we have to find out whether the sum of a and b will be even or odd.
Let us draw a table for all the possible cases for the evenodd property of a and b, and the corresponding property of the sum (a+b):
Thus we see that:
If a and b are both odd or both even, then the sum (a+b) will be even
But if one out of a and b is even, and the other is odd, the sum (a+b) will be odd.
So, we will need to determine the evenodd property of both a and b in order to be in a position to say whether the sum (a+b) is odd or even.
Step 3: Analyze Statement 1
Let’s analyze statement I:
a = k^{3} + 3k^{2} + 3k + 6
As we can see the value of a depends on the value of k. There can be two cases for the value of k – k can be either odd or even:
1. If k is odd:
This means, k^{3} = Odd (Odd*Odd*Odd = Odd; Think: 3*3*3 = 27)
Similarly, k^{2} will also be odd
So, 3k^{2} = Odd (Since 3 is odd and Odd*Odd = Odd)
Also, 3k = Odd
So, we get that
a = Odd number + odd number + odd number + even number
Think of easier numbers of the type of the right hand side of the above equation:
1 + 1 + 1 + 2 = 5, which is odd
This means that the RHS of the above equation will be an odd number
So, a is an odd number
So, if k is an odd number, a will be an odd number.
2. If k is even:
This means, k^{3} = Even (Even*Even*Even = Even; Think: 2*2*2 = 8)
Similarly, k^{2} will also be even
So, 3k^{2} = Even (Odd*Even = Even)
Also, 3k = Even
So, we get that
a = Even number + even number + even number + even number = Even number
So, a is an even number
Thus, if k is an even number, “a” will be an even number.
To conclude,
If k is odd => a is odd
If k is even => a is even
Thus, the value of a can be even or odd, depending on the value of k. This step fails to give us a definite answer whether a is an even number or odd number. Also, it does not give us any information about the values of b.
So, we can’t determine whether the expression a + b will be even or odd.
So, Statement 1 alone is not sufficient to arrive at a unique answer.
Step 4: Analyze Statement 2
Moving on, let’s analyze statement II:
b = (k^{2} + 4a +5) …………… (2)
Now, the value of b depends on the values of a and k.
If the values of a and k are independent of each other, then, we will have to consider 4 cases for the value of b:
Thus, we see that:
i) When k is even, b is odd
ii) When k is odd, b is even
iii) The evenodd property of b doesn’t depend on the value of a
Now, it is possible that the values of k and a are dependent on one another.
For example, if you were given that k = 8a
This would mean that irrespective of whether a is even or odd, k will always be even.
In that case, you would have been able to determine for sure that b will always be odd (though you would still be clueless about whether a is odd or even).
Or, if you were given that a = 8k, you would have been able to determine that a is even no matter what the value of k.
We are not given any information about the relationship, if any, between k and a. Neither are we given any other clue about the evenodd property of a.
Since this statement doesn’t tell us whether:
i) k is even or odd
ii) a is even or odd
iii) a and k are independent or not,
we will not be able to determine whether the sum (a+b) is even or odd.
So, Statement 2 alone is not sufficient to arrive at a unique answer
Step 5: Analyze Both Statements Together (if needed)
Since the individual analysis of the statements does not yield us any significant result, let’s analyse both of them together:
From Statement 1:
If k is odd => a is odd
If k is even => a is even
From Statement 2:
When k is even, b is odd
When k is odd, b is even
Combining the two statements, we get the following table:
Thus, we see that irrespective of the value of k, the sum (a+b) is always odd.
Thus, we have been able to arrive at a unique solution by combining the two statements.
Answer: Option (C)
If x and y are integers, is y even?
(1) (x + 2) * (y^{2} + 7) is even
(2) (x^{3} + 8) * (y^{2} 4) is even
Step 1 and 2: Understand the question and draw inferences
We are given that x and y are integers, and we are asked if y is even. Since we do not have any more information to draw inferences from, let’s move straight to Statement 1.
Step 3: Analyze Statement 1
(x + 2) * (y^{2} + 7) is even
The product of two numbers is even if at least one of the two numbers is even.
Thus, three possibilities arise:
Statement 1 holds true for each of these 3 cases. Thus, we cannot ascertain for sure if y is odd or even.
Therefore, Statement 1 alone is not sufficient to answer the question.
Step 4: Analyze Statement 2
(x^{3} + 8) * (y^{2} 4) is even
Again, the product of two numbers is even if at least one of the two numbers is even.
So, three possibilities arise for this statement as well:
Statement 2 holds true for each of these 3 cases. Thus, we cannot ascertain for sure if y is odd or even.
Therefore, Statement 2 alone is not sufficient to answer the question.
Step 5: Analyze both statements together (if needed)
We have seen that
Statement 1 holds true if:
i) x is even, y is even or odd
ii) x is odd, y is odd
Statement 2 holds true if:
i) x is even, y is even or odd
ii) x is odd, y is even
Both Statements will be true at the same time only if:
x is even, y is even or odd.
Thus, even after combining both the statements, we have not been able to ascertain the evenodd nature of y.
Thus, both the statements together are not sufficient to answer the question.
Correct Answer Choice: Option E
If x, y, and z are positive integers, where x is an odd number and z = x^{2} + y^{2} + 4. Is y^{2} divisible by 4?
(1) Z = 8k 3 where k is a positive integer
(2) When (zx+1) is divided by 2, it leaves a remainder.
Steps 1 & 2: Understand Question and Draw Inferences
We are given that x, y and z are positive integers.
Also, x is odd
And,
z= x^{2} + y^{2} + 4
By rearranging the terms, we get:
y^{2} = z – x^{2} – 4 …………. (1)
The question asks us whether y^{2 }is divisible by 4 or not. If y is divisible by 2, then y^{2} will be divisible by 4; else it will not be.
And if y is divisible by 2, this means that y is even.
Thus, the question is indirectly asking us: is y even?
y will be even if the right hand side of Equation 1 is even. So, let’s analyze the condition in which this will be so.
Now, we are given that x is an odd number. This means that x^{2} is also odd (Odd * Odd = Odd)
So, in the part (x^{2} – 4), we are subtracting an even number from an odd number. Think of what happens when we subtract 4 from 9. We get 5, which is an odd number. So, here too, the result of subtracting 4 from x^{2} will be an Odd number (Please note that the result may be positive or negative. We are not concerned with that. All that we want to know is whether the result will be even or odd)
So, from Equation 1 we get:
y^{2 } = z – Odd number . . . (2)
Now,
Even +/Odd = Odd (Think: 4 + 1 = 5)
And, Odd +/Odd = Even (Think: 3 + 1 = 4)
Thus, from equation (2)
It is clear that y^{2 }will be even only if z is an odd number
Step 3: Analyze Statement 1
Statement (1) says:
z = 8*k – 3
Now, irrespective of whether k is even or odd,
8 * k = even number
ð 8k – 3 = Even number odd number = Odd Number (Think: 8 – 3 = 5)
So, the analysis of statement (1) tells us that z is an odd number.
Since we have been able to determine that z is an odd number, this means that y is an even number.
Thus, Statement (1) alone is sufficient to answer the question: is y even?
Step 4: Analyze Statement 2
When the expression (zx+1) is divided by 2, we get a remainder. This tells us that (zx+1) is an odd number. So,
z – x + 1 = odd
This means that,
z – x = Odd number – 1 = Odd number – odd number = Even number (Think: 5 – 3 = 2)
So, z = x + Even number
Now, we are given that x is an odd number
So, z = Odd number + Even number = Odd number (Think: 1 + 2 = 3)
Thus, z is an odd number
Thus, Statement 2 alone is sufficient to answer the question: is y even?
Step 5: Analyze Both Statements Together (if needed)
Since we got unique answers in step 3 and 4, so this step is not required
Answer: Option (D)
If A is a positive integer, then which of the following statements is true?
1. A^{2} + A 1 is always even.
2. (A^{4}+1)(A^{4}+2) + 3A is even only when A is even.
3. (A1)(A+2)(A+4) is never odd.
Step 1: Question statement and Inferences
We are given that A is a positive integer. A may be odd or even.
Step 2: Analyzing the given statements
1. A^{2} + A 1 is always even
Case (i) A is Odd
We don’t even need to consider the case when A is even, because Statement 1 has already been proven FALSE.
2. (A^{4}+1)(A^{4}+2) + 3A is even only when A is even
Case (i): A is odd
Now, an odd number multiplied with itself any number of times will give an odd product
Thus, when A is odd, (A^{4}+1)(A^{4}+2) + 3A is odd
Case (ii): A is even
3A is even (Odd × Even = Even)
An even number multiplied with itself any number of times will give an even product
Thus, when A is even, (A^{4}+1)(A^{4}+2) + 3A is even
Thus, Statement 2 is TRUE.
3. (A1)(A+2)(A+4) is never odd
Case (i): A is odd
Thus, when A is odd, (A1)(A+2)(A+4) is even
Case (ii): A is even
Thus, when A is even, (A1)(A+2)(A+4) is even
Thus, Statement 3 is TRUE.
Answer: Option (E)
If a, b, c, d, and e are integers and the expression (ab^{2}c^{2} /d^{2}e) gives a positive even integer, which of the following options must be true?
I. abc is even
II. a/e is positive
III. a/d^{2} is positive
Step 1: Question statement and Inferences
We are given that a, b, c, d, and e are integers.
Also, it is given that
ab^{2}c^{2} /d^{2}e = positive even number . . .. (1)
We can also write this expression as:
ab^{2}c^{2} = (Positive Even number)*(d^{2}e)
When you multiply any number, whether it is even or odd, with an even number, the product is always even.
So, we can deduce that:
Also, equation (1) can be written as:
(b^{2}c^{2} /d^{2}) * (a/e) = positive even number
Now, we know that the product of two numbers can be positive only if both are positive or both are negative: (Think; 2*3 = 6 OR 2 * 3 = 6)
And in the above expression, (b^{2}c^{2} /d^{2}) is always positive since even powers of an integer are always positive.
Since the first number (b^{2}c^{2} /d^{2}) is always positive, the second number (a/e) also needs to be positive.
(Note that, here it is possible that a and e both are negative, but the overall expression (a/e) is definitely positive)
So, we know that a/e is positive.
Step 2: Finding required values
Now, let’s move on to the analysis of the options:
I. abc is even.
As we know, the expression (ab^{2}c^{2}) is even and it can be written as:
ab^{2}c^{2} = a*b*b*c*c
Now, since the above expression yields an even number, it tells us that at least one of the integers a, b, or c is an even number. (Think 2 * 3 * 5 = 30)
Now, if at least one of the integers a, b, or c is an even number then the product (a*b*c) will also be an even number.
So, this statement is always true.
II. a/e is positive
We already proved that a/e is positive.
So, this statement is always true. III. a/d^{2} is positive
In the expression a/d^{2} since d^{2} is a square form, it is always positive. However, the question doesn’t state whether a is positive or negative. So, we can’t clearly say whether a/d^{2} is positive or negative.
So, we can’t say that this statement is always true.
Step 3: Calculating the final answer
Hence, the options that are always true are I and II. So, the correct answer choice is choice C.
Answer: Option (C)
Take Away
(2) * (1) * (3) = 6
(3) * (4) * (5) = 60
If r and s are positive integers, and r^{2} + r/s is an odd integer, which of the following cannot be even?
Step 1: Question statement and Inferences
Given: r and s are positive integers
r^{2} + r/s is odd
To Infer: Let’s try to infer whether r and s are even or odd.
Let’s first consider the possibility that r is odd.
Case (i): r is odd
> r^{2} is odd (Odd × Odd = Odd)
Now, r^{2} + r/s = Odd
> Odd + r/s = Odd
> r/s ^{ }= Even (Odd Odd = Even)
> Odd/s = Even
> s × Even = Odd
Not possible! When one of the numbers being multiplied is even, the product is always even
Conclusion: r cannot be odd
> r must be even
Let’s also see if we can determine whether s is even or odd.
We know now that r is even
> r^{2} is even (Odd × Odd = Odd)
Now, r^{2} + r/s = Odd
> Even + r/s = Odd
> r/s ^{ }= Odd (Odd Even = Odd)
> Even/s = Odd
> s × Odd = Even
> s is Even
Conclusion: Both r and s are even integers
Step 2: Analyzing the given options
Both r and s are even
> 3r is even (Odd × Even = Even)
2s is even (Even × Even = Even)
> 3r + 2s is even (Even + Even = Even)
Both r and s are even
> r1 is odd (Even Odd = Odd)
s + 2 is even (Even + Even = Even)
> (r1)(s+2) is even (Odd × Even = Even)
Both r and s are even
An even number multiplied with itself any number of times will give an even product
> r^{s+1} is even
s^{r } is even
> r^{s+1} + s^{r }is even
r is even
> r^{3} is even
> r^{3} + 3 is odd
We don’t need to analyze Option E since we have already found the expression that cannot be even.
Step 3: Calculating the final answer
Among the given options, Option D cannot be even
Answer: Option (D)
Which one of the following is odd?
Step 1:
8 × 4 = 32
6 × 7 = 42
9 × 6 = 54
Step 2:
Only 11 × 3 = 33 is odd. Rest are even.
Which one of the following is even?
Step 1:
3 x 9 = 27
5 x 7 = 35
5 x 3 = 15
Step 2:
Only 9 × 8 = 72 is even. Rest are odd.
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