Question

There are 10 rolls of film in a box and 3 are defective. two rolls are to be selected, one after the other. what is the probability of selecting a defective roll followed by another defective roll?

Answer 1

1 / 15 = 0.06666666666....

Explanation:

Total Number of Rolls = 10

Number of Defective Rolls = 3

Probability = Number of Defective Rolls / Total Number of Rolls

Probability = 3 / 10

Now there is one less defective roll.

Total Number of Rolls = 10 - 1

Number of Defective Rolls = 3 - 1

Total Number of Rolls = 9

Number of Defective Rolls = 2

Probability = Number of Defective Rolls / Total Number of Rolls

Probability = 2 / 9

Multiply the two probabilities together to find the overall probability.

3 / 10 * 2 / 9

3 * 2 = 6

10 * 9 = 90

6 / 90

Simplify

6 / 90 = 3 / 45 = 1 / 15 = 0.06666666666....