The function $\displaystyle \displaystyle f$ and its inverse $\displaystyle \displaystyle f^{-1}$ are continuous. If $\displaystyle \displaystyle f(0) = 0$, find $\displaystyle \displaystyle\int^5_0 f(x)\ dx + \int^{f(5)}_0 f^{-1}(t)\ dt$

I really have no idea how to proceed.

Answer: 5f(5)