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All questions of Probabilistic Reasoning for UCAT Exam

A die is rolled twice. What is the probability of getting a sum equal to 9?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Total number of outcomes possible when a die is rolled = 6 (∵ any one face out of the 6 faces)
  • Hence, total number of outcomes possible when a die is rolled twice, n(S) = 6 x 6 = 36
E = Getting a sum of 9 when the two dice fall = {(3,6), (4,5), (5,4), (6,3)}
  • Hence, n(E) = 4

A bag contains 2 yellow, 3 green and 2 blue balls. Two balls are drawn at random. What is the probability that none of the balls drawn is blue?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Total number of balls = 2 + 3 + 2 = 7
► Let S be the sample space.
  • n(S) = Total number of ways of drawing 2 balls out of 7 = 7C2
► Let E = Event of drawing 2 balls, none of them is blue.
  • n(E) = Number of ways of drawing 2 balls from the total 5 (= 7-2) balls = 5C2
    (∵ There are two blue balls in the total 7 balls. Total number of non-blue balls = 7 - 2 = 5)

Can you explain the answer of this question below:

A bag contains 4 black, 5 yellow and 6 green balls. Three balls are drawn at random from the bag. What is the probability that all of them are yellow?

  • A:

  • B:

  • C:

  • D:

The answer is A.

Divey Sethi answered
Total number of balls = 4 + 5 + 6 = 15
Let S be the sample space.
  • n(S) = Total number of ways of drawing 3 balls out of 15 = 15C3
Let E = Event of drawing 3 balls, all of them are yellow.
  • n(E) = Number of ways of drawing 3 balls from the total 5 = 5C3
    (∵ there are 5 yellow balls in the total balls)

[∵ nCr = nC(n-r). So 5C3 = 5C2. Applying this for the ease of calculation]

What is the probability of selecting a prime number from 1,2,3,... 10 ?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Nikita Singh answered
Total count of numbers, n(S) = 10

Prime numbers in the given range are 2,3,5 and 7
Hence, total count of prime numbers in the given range, n(E) = 4

There are 15 boys and 10 girls in a class. If three students are selected at random, what is the probability that 1 girl and 2 boys are selected?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Let S be the sample space.
  • n(S) = Total number of ways of selecting 3 students from 25 students = 25C3
Let E = Event of selecting 1 girl and 2 boys
  • n(E) = Number of ways of selecting 1 girl and 2 boys
15 boys and 10 girls are there in a class. We need to select 2 boys from 15 boys and 1 girl from 10 girls
Number of ways in which this can be done: 
15C2 × 10C1
Hence n(E) = 15C2 × 10C1

A randomly selected year is containing 53 Mondays then probability that it is a leap year
  • a)
    2 / 5
  • b)
    3 / 4
  • c)
    1 / 4
  • d)
    2 / 7
Correct answer is option 'A'. Can you explain this answer?

The correct option is A 
 
  • Selected year will be a non leap year with a probability 3/4
  • Selected year will be a leap year with a probability 1/4
  • A selected leap year will have 53 Mondays with probability 2/7
  • A selected non leap year will have 53 Mondays with probability 1/7
  • E→ Event that randomly selected year contains 53 Mondays
P(E) =  (3/4 × 1/7) + (1/4 × 2/7)
P(Leap Year/ E) = (2/28) / (5/28) = 2/5 
 

A card is randomly drawn from a deck of 52 cards. What is the probability getting either a King or a Diamond?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Total number of cards = 52

Total Number of King Cards = 4
Total Number of Diamond Cards = 13
Total Number of Cards which are both King and Diamond = 1
Here a card can be both a Diamond card and a King. Hence these are not mutually exclusive events. (Reference : mutually exclusive events) . By Addition Theorem of Probability, we have P(King or a Diamond) = P(King) + P(Diamond) – P(King and Diamond)

When two dice are rolled, what is the probability that the sum is either 7 or 11?
  • a)
     
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Total number of outcomes possible when a die is rolled = 6 (? any one face out of the 6 faces)

Hence, total number of outcomes possible when two dice are rolled = 6 × 6 = 36

To get a sum of 7, the following are the favourable cases.
(1, 6), (2, 5), {3, 4}, (4, 3), (5, 2), (6,1)

=> Number of ways in which we get a sum of 7 = 6
To get a sum of 11, the following are the favourable cases. (5, 6), (6, 5) => Number of ways in which we get a sum of 11 = 2
Here, clearly the events are mutually exclusive events. By Addition Theorem of Probability, we have P(a sum of 7 or a sum of 11) = P(a sum of 7) + P( a sum of 11)

5 coins are tossed together. What is the probability of getting exactly 2 heads?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Total number of outcomes possible when a coin is tossed = 2 (? Head or Tail)
Hence, total number of outcomes possible when 5 coins are tossed, n(S) = 25

E = Event of getting exactly 2 heads when 5 coins are tossed
n(E) = Number of ways of getting exactly 2 heads when 5 coins are tossed = 5C2

John and Dani go for an interview for two vacancies. The probability for the selection of John is 1/3 and whereas the probability for the selection of Dani is 1/5. What is the probability that only one of them is selected?
  • a)
    3/5
  • b)
    2/5
  • c)
    1/5
  • d)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Sameer Rane answered
Let A = the event that John is selected and B = the event that Dani is selected.

Given that 𝑃(𝐴) = 1/3 and 𝑃(𝐵) = 1/5

We know that A is the event that A does not occur and B is the event that B does not occur

Probability that only one of them is selected

Three coins are tossed. What is the probability of getting at most two tails?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Here S = {TTT, TTH, THT, HTT, THH, HTH, HHT, HHH}

Let E = event of getting at most two heads.

Then E = {TTT, TTH, THT, HTT, THH, HTH, HHT}.

Therefore  P(E) = n(E)/n(S) = 7/8.

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