Question

The probability all of the students in this class will be present and on time is 3/10. The probability all of the students are present is 2/5. What is the probability that everyone is on time given that everyone is present? Give your answer as a simplified fraction.​

Answer 1

3/4

Explanation:

A: students present
B: students on time

P(all students present and on time) = P(A and B) = 3/10
P(all students present) = P(A) = 2/5

P(A and B) = P(A).P(B|A) where P(B|A) is the probability of everyone being on time given that everyone is present

So P(B|A) = P(A and B) /P(A) = 3/10 ÷ 2/5 = 3/10 * 5/2 = 15/20 which can be reduced to 3/4 by dividing numerator and denominator by 5