Numerical Methods: Euler to Runge–Kutta · Challenge problem

Heun's method (global error $\approx Ch^2$) is applied to $y' = y$, $y(0) = 1$ over $[0, 1]$, once with $h = 1$ and once with $h = \dfrac{1}{2}$. Combine the two estimates of $y(1)$ by Richardson extrapolation. Express your answer as a common fraction.