Pipe 1 can fill a reservior in 45 hours. pipes 1 and 2 together can fill it in 30 hours. pipes 2 and 3 together can fill it in 40 hours, in how much time can pipe 2 alone and in how much time can pipe 3 alone fill the reservior?

Now let's say hmmm
p1 = pipe1   p2 = pipe2    and p3 = pipe3

so, we know p1 can do the whole shebang in 45hours
that simply means, that in 1hr, percentage wise, p1 can do really (1/45)th of the job

ok... now, if we have p1 and p2 working together, they can do the whole thing in  30hrs, so, in 1hr, they both working, have done only (1/30)th of the job, so, what's p2's rate? well



that simply means, p2 can do the whole job in 90 hours... notice, 90 is 45*2, that just means p2 is twice as slow as p1

now.. we know p2 and p3 working together can do the job in 40hrs, what's p3's rate?

well 

so p3, can do the whole shebang in 72hrs then