The management at a fast-food outlet is interested in the joint behavior of the random variables y1, defined as the total time between a customer's arrival at teh store and departure from the service window, and y2, the time a customer waits in line before reaching the service window. because y1 includes the time a customer waits in line, we must have y1 > y2. the relative frequency distribution of observed values of y1 and y2 can be modeled by the probability density function.

f(y1,y2) = e^(-y1), 0 < = y2 < = y1 < infinity,
0, elsewhere
with time measure in minutes find.

mostly just interested in how to set these up and why? i am assuming its not as it is normally.
a.) p(y1 < 2, y2 > 1)
b.) p(y1 > = 2y2)
c.) p(y1 - y2 > = 1) (notice that y1 - y2 denotes the time spent at the service window