Numerical Methods: Euler to Runge–Kutta · Medium problem

A cup of coffee cools according to Newton's law of cooling $T' = -0.3(T - 70)$ (T in degrees Fahrenheit, t in minutes), $T(0) = 200$. Using Euler's method with step $h = 1$ minute, after how many whole minutes does the Euler estimate of the temperature first drop below $120$ degrees?