Answer
The average velocity of the boat in still water is $21$ mph and that of the current is $3$ mph.
Work Step by Step
Step 1. Assume the average velocity of the boat in still water is $x$ mph and that of the current is $y$ mph.
Step 2. When traveling with the stream, the total average velocity is $x+y$; thus $1.5(x+y)=36$ or $x+y=24$
Step 3. When traveling against the stream, the resulting average velocity is $x-y$; thus $2(x-y)=36$ or $x-y=18$
Step 4. Adding the two equations, we get $2x=42$; thus $x=21$ and $y=3$.
