Maria takes a boat upstream for 9 hours. Her return trip downstream only took 6 hours. If the river she is on has a current of 1 mile per hour, how fast does her boat move in still water?

5 mph

Explanation:

Let her speed in still water be v (mph) relative to water

When moving upstream, her speed relative to ground is v - 1, her time is 9 hours, so her distance traveled is 9(v - 1)

When moving downstream, her speed relative ground is v + 1, her time is 6 hours, so her distance is 6(v + 1)

Since both distances upstream and downstream are the same, we can equate the 2 above

9(v - 1) = 6(v + 1)

9v - 9 = 6v + 6

3v = 6 + 9 = 15

v = 5

So her speed in still water is 5 mph