Numerical Methods: Euler to Runge–Kutta · Medium problem

A tank holds $100$ L of brine (volume held constant by equal in/out flow) with $50$ kg of salt dissolved. Fresh water flushes the tank so the salt amount obeys $S' = -\dfrac{1}{20}S$ (S in kg, t in minutes), $S(0) = 50$. Using Euler's method with $h = 4$ minutes for two steps, estimate the salt concentration (in kg/L) at $t = 8$ minutes. Express your answer as a common fraction.