An athlete swims the length $\mathrm{????}$ of a pool in a time ${\mathrm{????}}_1$ and makes the return trip to the starting position in a time ${\mathrm{????}}_2.$ If she is swimming initially in the positive $\mathrm{????}-$direction, determine her average velocities symbolically in $($a$)$ the first half of the swim, $($b$)$ the second half of the swim, and $($c$)$ the round trip. $($d$)$ What is her average speed for the round trip?