Two ink dispensers discharge ink into a color mixer. The first one can fill it in 21 minutes, whereas the second can fill it in 7 minutes. Both them are opened at the same time, but the second ink dispenser is turned off after 4 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 $T/21 + 4/7$ = 1. Solving, T = 9 mins.