Four Hundred Pages by Time t

Each page of a 400-page book takes Tesfa an Exponential(rate 0.2) number of minutes, independently across pages.

Using the central limit theorem, approximate the time $t$, in whole minutes, at which $P(\text{total reading time} \leq t)$ reaches $0.975$. Use $\Phi^{-1}(0.975) = 1.96$.

Answer format: enter an integer.