Question

At Cornell School, all first year students must take chemistry and math. Suppose 25% fail in chemistry, 18% fail in math, and 9% fail in both. Suppose a first year student is selected at random. What is the probability that student selected failed at least one of the courses?

(a) 0.22 

(b) 0.34 

(c) 0.45 

(d) None of these

Answer 1

Correct option (b) 0.34

Explanation:

To Solve this problem use Additive law of Probability. 

P(A∪B)=P(A)+P(B)-P(A∩B)

P(C) = 0.25 

P(M) = 0.18 

P(C∩M) = 0.09 

P(at least one) = P(C or M) = P( C ∪ M) 

= 0.25 + 0.18 - 0.09 = 0.34