Two ink dispensers discharge ink into a color mixer. The first one can fill it in 36 minutes, whereas the second can fill it in 9 minutes. Both them are opened at the same time, but the second ink dispenser is turned off after 7 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 $T/36 + 7/9$ = 1. Solving, T = 8 mins.