The probability that a battery will last 10 hr or more is 08

The probability that a battery will last 10 hr or more is 0.8, and the probability that it will last 15 hr or more is 0.11. Given that a battery has lasted 10 hr, find the probability that it will last 15 hr or more. (Round your answer to three decimal places.)

Solution

Let A = the battery lasts 10 hours or more, so P(A)=.8

B = the batter lasts 15 hours or more so P(B)=.11

B?A gives that P(A?B)=P(B)

We need P(B|A)=P(A?B)/P(A)  

= P(B)/ P(A) 0.11/0.8

=0.1375

The required P= 0.138   ( three decimals)