Speed on still water

$t$ = time it takes the motorboat to travel $36$ miles downstream on river $A$ and time it takes to travel $16$ miles upstream on river $B$ .

$v$ = speed of the motorboat in still water

working formula: $speed=$ $\frac{distance}{time}$

on River $A$ ,

$v + 6=$ $\frac{36}{t}$ or $t=\frac{36}{v+6}$

on River $B$ ,

$v - 4=$ $\frac{16}{t}$ or $t = \frac{16}{v-4}$

Since the time it travels $36$ miles downstream is equal to the time it takes to travel $16$ miles upstream is equal, equate $t$ and $t$ to solve for $v$ .

$\frac{36}{v+6} = \frac{16}{v-4}$

$v = 12$ miles per hour