As a punishment for something naughty that we did, my little brother and I have to whitewash both sides of a fence. We start at the same time, and we each work at a constant rate.
If we each whitewash one side, I'll finish in 2 hours and my brother will finish in 4 hours. But I'm a nice kid, so after I finish my side, I go around to the other side and help my brother finish his side. From the time I start helping him, how many minutes does it take us to finish the job?
When you finish your side, your brother will still have two hours left.
In one hour you can finish 1/2 of a side and your brother can fininsh 1/4 of aside
So...in one hour working together you both can finish 1/2 + 1/4 = 3/4 of a side
Take the reciprocal of this = 4/3
So....it will take 4/3 of an hour = 1hr 20 min = 80 min to finish once you start helping him