Probability: Solved Examples- 2 | CSAT Preparation - UPSC PDF Download

Example 1: An urn contains 10 white and 15 black balls. Two balls are drawn in succession without replacement. What is the probability that first is white and second is black?

Solution: Let A = Getting a white ball in the first draw B = Getting a Black Ball in the second draw. Required Probability = Probability of getting a white ball in the first draw and a black ball in second draw = P(A and B) = P (A ∩B) = P (A) .P(B/A)

P(A) = (¹⁰C₁/²⁵C₁) = (10/25) = (2/5) and P(B/A) = Probability of getting a black ball in the second draw
when a white ball has already been in first draw  = (¹⁵C₁/²⁴C₁) = (15/24) = (5/8)
⇒ Required probability

Example3: Hunar wrote two sections of CAT paper ; Verbal and QA in the same order. The probability of her passing both sections is 0.6. The probability of her passing the verbal section is 0.8. What is the probability of her passing the QA section given that she has passed the Verbal section?

Then by the Law of total probability: 

P(A) = P(A | X)-P(X) + P(A | Y) P(Y ) + P(A/Z).P(Z).

= (4/ 10).(1/ 3) + (1/ 6) . (1/ 3) + (3/8) .(1/3) = 113/360 .