The table shows the volume of water, $V$, in a tank at different times, $t$, where $P$ is a constant representing the initial volume. The relationship between time and volume is linear. What is the rate of change of the volume of water in the tank, in liters per minute?

Time (minutes), $t$	Volume (liters), $V$
10	P + 20
25	P + 5
40	P - 10