Question 42785
We could consider the speed of the boat as x and the stream as y. The stream takes the boat forward or backwards, so we have two equations: x + y = 72 (the downstream takes the boat forward) and x - y = 36.
*[invoke linear_substitution "x", "y", 1, 1, 72, 1, -1, 36 ]
If y=18, the "time" we took is y/6 = 3 hours, and x is the speed of the boat in three hours. The answer we want is better given in an hour period, so our answer is {{{54/3 = 18}}} mph.