A supplier delivers an order for 20 electric toothbrushes to a store. By accident,three of the electric toothbrushes are defective. What is the probability that the first two electric toothbrushes sold are defective?
A)3/20 or 0.15
B)3/17 or 0.176
C)1/4 or 0.25
D)6/380 or 0.01579

Question

Quizplus · Accepted Answer

The probability that the first electric toothbrush sold is defective is 3/20. 
After the first one is sold, there will be 19 toothbrushes left, including 2 defective ones. So, the probability that the second electric toothbrush sold is also defective is 2/19. 
The probability of both events happening together (the first two toothbrushes sold being defective) is the product of their probabilities: 
P(defective,defective) = (3/20) x (2/19) = 6/380 or 0.01579. 
Therefore, the correct answer is D.

A Supplier Delivers an Order for 20 Electric Toothbrushes to a Store