An oil tank is drained of oil such that if V kl of oil are in the tank t minutes after draining commences, then dV/dt = 5t-40. The initial full tank is emptied in 4 mins.

determine the number of kilo lit of oil drained in the fourth minutes.

This is really annoying. I set the equation up to be (5t-40)^2 /10 and solve from the range f(4) - f(0) but I'm not getting the answer.

However, if I set f(4) - f(3) I get the answer. Why is the range from 4 to 3?

Also, what is the capacity of the tank?