$\mathrm{An}$ athlete leaves one end of a pool of length $L$ at $t=0$
S and arrives at the other cnd at time $t_{1}$, She swims hack and arrives at the starting position at time $t_{2}$. If she is swimming initially in the positive $x$ direction, determine her average wrioritics symbolically in (a) the lirst hatr of the swim,
(b) the second hall' of the swim, and (c) the round trip.
(d) What is her average speed for the round trip?