What exactly do you mean by Large pipe = l? Is l the time the large pipe takes to fill the tank? In what, minutes, hours? Is it the portion of the tank that gets filled after one hour with the large pipe running? What is it?
I suggest the following: let
l = the time the large pipe takes to fill the tank, in minutes
s = the time the small pipe takes to fill the tank, in minutes
The large pipe being able to fill the tank in 2 hours less than the small one immediately gives (remember we are working in minutes, so 2h = 120mins). To get the second condition, think of it as follows: if the large pipe takes l minutes to fill the tank, then (assuming they pour water at constant rate) it will fill of the tank in a single minute. So in 80 minutes, it will fill of the tank. Same goes for s, and you finally get:
(1 being one filled tank)
Try to work these out now, and you'll see a quadratic equation appearing along the way, which may explain your thread title