Question

A box contains 30 widgets, 4 of which are defective. If 4 are sold at random, find the probability that (a) all are defective (b) none are defective.

Answer 1

a. The probability that all are defective is 0.0003160493827

b. Probability that none are defective is 0.99968395

Explanation:

Given that 4 of the 30 widgets contained on the box are defective, n = 4. The probability of picking a defective widget is p = 4/30 = 2/15.

Now, P(X = a) = (nCa)P^n(1 - P)^(n - a).

a. To find the probability that all are defective, we want to find P(X = 4)

= (4C4) × (2/15)^4 × (1 - 2/15)^(4 - 4)

= 1 × (2/15)^4 × 1

= 0.0003160493827

b. Probability that none are defective.

This is the same as saying (1 minus the probability that all are defective).

P = 1 - 0.0003160493827

= 0.99968395