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.

a. The probability that all are defective is 0.0003160493827

b. Probability that none are defective is 0.99968395

Step-by-step 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