Are we meant to assume from your post title that the time follows an exponential distribution? If so, then the mathemtaical translation of what you have written, namely, Pr(X = 2) = 0.4, Pr(X = 4) = 0.6 and Pr(X = 8) = 0.85 makes no sense.

Surely you mean something like

40% of the returns are completed in 2 hours or less, 60% are completed in 4

hours or less and 85% are completed in 8 hours or less.

Until you have clarified the situation there's nothing more that can be done.