Ratios and Rates · Challenge problem

A graph shows litres pumped (up) against minutes (across) for two pumps. Each pump's graph is a straight line starting at $(0, 0)$. Pump A's line passes through $(3, 12)$ and Pump B's line passes through $(5, 15)$. Both pumps start at the same moment. After how many minutes has Pump A pumped $20$ litres more than Pump B?