Finding the average of two *speeds* requires a bit more care than finding the average of other quantities. Consider the following runner who completes a $\SI[per-mode=symbol]{180}{\meter}$ run with these speeds:

Since the "average" speed is $\SI[per-mode=symbol]{5}{\meter\per\second}$, this entire run should take $\frac{180}{5}=36$ seconds, right?