Two ink dispensers discharge ink into a color mixer. The first one can fill it in 40 minutes, whereas the second can fill it in 10 minutes. Both them are opened at the same time, but the second ink dispenser is turned off after 5 minutes. What is the total time required to fill the color mixer cistern?
If the total time is T, the sum of works done by the ink dispensers are $T/40 + 5/10$ = 1. Solving, T = 20 mins.