# How do i calculate the probability

I have this question but i have difficult to understand and which probability rules i use

Assume that an old used car fails to start 10% of the time. The other 90% of the time it drives for a time period which is uniformly distributed in the interval 20 to 30 minutes. In this interval all subintervals of time of equal width between 20 to 30 minutes are equally likely.

What is the probability that the car drives for at least 10 minutes given that it starts?

any suggestion !!

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If it starts, it will go for at least 20 minutes, which means it will go for at least 10 minutes. Hence the probability is 1. –  Calvin Lin Jan 15 '13 at 2:10
Yeah, I agree with @CalvinLin, given that the car starts, the minimum it will run for is 20 minutes so the problem is trivially solved. –  Inquest Jan 15 '13 at 2:19
Identical question is part of an on-going assignment Question 6; Assignment I; COMP 5107; due 17 Jan 2013. –  Douglas S. Stones Jan 15 '13 at 4:26
@user58310 Which grade did you receive for the assignment? –  Did Jan 21 '13 at 8:26