two buses, A and B, operate on a route. A person arrives at a bus stop on this route at time 0. Let X and Y be the arrival times of bus A and bus B respectively, at this stop afterward. Suppose that X and Y are independent and have the density functions, respectively, by:

f1(x) = 1/a, if 0<x<a

0 otherwise

f2(y) = 1/b, if 0<y<b

0 otherwise

where a,b>0 are constants. what is the probablity that bus B will arrive before bus A?

THANKS