Question 54953
Let x = speed of the boat in still water
y = speed of the current of the river


x+y = rate downstream
x-y = rate upstream


Distance = Rate * Time, so 
{{{Rate = Distance/Time}}}


x+y={{{36/3}}}=12  Downstream
x-y={{{36/9}}}=4  Upstream


Add these two equations together:
2x = 16, so x = 8 mph in still water


x+y = 12, where x = 8, y = 4 mph current.


R^2 at SCC