Ratio of partial sums

Let $X_1, X_2, \dots, X_{40}$ be i.i.d. positive random variables with $\mathbb{E}[X_1] = 2$ and $\mathbb{E}[1/X_1] = 1$, and let $S_n = X_1 + \dots + X_n$. Compute $\mathbb{E}\!\left[\dfrac{S_{20}}{S_{40}}\right]$.