Answer
The speed of the boat in still water is 20 mph.
Work Step by Step
Let $x$ mph be the speed of the boat in still water.
By $Speed = \frac{Distance}{Time}$, and equating the time taken, we have,
Upstream time (against the current) = Downstream time (along with the current)
$\frac{48}{x - 4} = \frac{72}{x + 4}$
$48(x + 4) = 72(x - 4)$
$48x + 192 = 72x - 288$
$24x = 480$
$x = 20$
The speed of the boat in still water is 20 mph.
