A watch manufacturer has two factories (FA, FB) and 60% of their watches are made at FA. It is known that 10% of them are made at FA and 15% made at FB are defective. What is the probability that a selected defective watch was manufactured at FB?

Answer: Half of the defective watches are from factory FB.

Imagine that 100 watches are made at the two factories. 60 of them would be made at Factory FA and 40 of them would be made at Factory FB.

In Factory FA, 10% of the 60 would be defective. That is 6.

In Factory FB, 15% of the 40 would be defective. That is also 6.

So if you select one of the 12 defective watches, half would come from FA and half would come from FB.