Numerical Methods: Euler to Runge–Kutta · Medium problem

A tank with capacity $20$ L starts empty. Water flows in at a rate $r(t) = 5 - 0.5t$ L/min (t in minutes). Using Heun's (improved Euler) method with $h = 2$ minutes for two steps ($k_1 = r(t_n)$, $k_2 = r(t_n+h)$, $V_{n+1} = V_n + \dfrac{h}{2}(k_1+k_2)$), estimate the volume in the tank at $t = 4$ minutes, and find how many liters of capacity remain at that time.