Question 1094371

Traveling at full speed on a river, it takes 4 hours for a motorboat to travel 18 miles upstream and 21 miles downstream.  If the current's speed is 2 mph, what is the maximum speed of the boat in still water?
<pre>Correct answer: {{{highlight_green(matrix(1,6, "Boat's", "speed,", in, still, "water:", highlight(matrix(1,2, 10, mph))))}}}