A motorboat is capable of traveling at a speed of 12 miles per hour in still water. On a particular day, it took
15 minutes longer to travel a distance of 4 miles upstream than it took to travel the same distance downstream. What was the rate of current in the stream on that day?
On an ordinary day, the boat can cover the distance of 4 miles in:
4 /12 =1/3 of an hour, or 1/3 x 60 = 20 minutes.
But, on some particular day, it took the boat: 20 + 15 = 35 minutes to travel that 4 miles.
So: 4 / x =35/60
x =4 / (35/60)
x =6 6/7 mph - the speed of the boat on that particular day.
12 - 6 6/7 = 5 1/7 mph - the rate of the current on that day.