David is deciding to buy either a new car or a used car that has been driven 10,000 miles. For each car, find the probability that he can drive it for an extra 20,000 miles in the following scenarios.

(a) The total mileage that both cars can be driven before they completely break down is an exponential random variable with mean 20 thousands miles.
(b) The total mileage that both cars can be driven before they completely break down is uniformly distributed over (0, 40,000).