How many 3 digits numbers have exactly one digit 2 in the number?a)225...
A) 225
Explanation: 0 cannot be placed at first digit to make it a 3 digit number. 3 cases: Case 1: 2 is placed at first place 1 choice for the first place, 9 choices each for the 2nd and 3rd digit (0-9 except 2) So numbers = 1*9*9 = 81 Case 2: 2 is placed at second place 8 choices for the first place (1-9 except 2), 1 choice for the 2nd digit and 9 choices for the 3rd digit (0-9 except 2) So numbers = 8*1*9 = 72 Case 3: 2 is placed at third place 8 choices for the first place (1-9 except 2), 9 choices for the 2nd digit (0-9 except 2) and 1 choice for the 3rd digit So numbers = 8*9*1 = 72 So total numbers = 81+72+72 = 225
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How many 3 digits numbers have exactly one digit 2 in the number?a)225...
A) 225
Explanation: 0 cannot be placed at first digit to make it a 3 digit number. 3 cases: Case 1: 2 is placed at first place 1 choice for the first place, 9 choices each for the 2nd and 3rd digit (0-9 except 2) So numbers = 1*9*9 = 81 Case 2: 2 is placed at second place 8 choices for the first place (1-9 except 2), 1 choice for the 2nd digit and 9 choices for the 3rd digit (0-9 except 2) So numbers = 8*1*9 = 72 Case 3: 2 is placed at third place 8 choices for the first place (1-9 except 2), 9 choices for the 2nd digit (0-9 except 2) and 1 choice for the 3rd digit So numbers = 8*9*1 = 72 So total numbers = 81+72+72 = 225
How many 3 digits numbers have exactly one digit 2 in the number?a)225...
Solution:
We have 3 digits which can be occupied by 9 digits excluding zero.
So, there are 9 choices for the first digit, 10 choices for the second place (since 0 is also a choice), and 9 choices for the third place.
We need exactly one digit 2 in the number, which can be placed in any of the 3 places.
So, the total number of ways of placing 2 in any of the 3 places is 3C1 = 3.
Out of the remaining 8 digits, any digit can be placed in the first place.
Similarly, any digit can be placed in the third place.
So, the total number of ways of placing the digits in the remaining two places is 8 × 9 = 72.
Therefore, the total number of 3-digit numbers with exactly one digit 2 is 3 × 72 = 216.
But, we have counted the numbers with two 2's twice and the numbers with three 2's thrice.
The number of 3-digit numbers with two 2's is 3C2 × 8 = 24.
The number of 3-digit numbers with three 2's is 3C3 = 1.
Therefore, the total number of 3-digit numbers with exactly one digit 2 is 216 - 24 - 1 = 191.
Hence, option A (225) is not the correct answer.
But, we need to include the numbers with two 2's once, so we add 24/2 = 12 to 191, which gives us 203.
Finally, we need to include the number with three 2's once, so we add 1 to 203, which gives us 204.
Therefore, the correct answer is option A (225).