Numerical Methods: Euler to Runge–Kutta · Medium problem

A radioactive sample decays via $y' = -0.5y$ (y in mg), $y(0) = 100$. Euler's method with $h = 1$ year gives $y(1) \approx 50$ mg and $y(2) \approx 25$ mg. Using linear interpolation between these two Euler estimates, find the estimated time (in years) at which the sample first reaches $40$ mg. Express your answer as a common fraction.