Answer:
20 mph.
Step-by-step explanation:
Let the speed of the current be x mph.
54 minutes = 54/60 = 0.9 hours and 18 minutes = 18/60 = 0.30 hours
Travelling against the current:
Speed = distance / time
x - 10 = d / 0.9
With the current:
x + 10 = d / 0.3
Subtract the second equation from the first:
0 - 20 = d/0.9 - d/0.3
-20 = (d - 3d) / 0.9
-20 * 0.9 = -2d
-2d = -18
d = 9
So x + 10 = 9/0.3 = 90/3
x + 10 = 30
x = 20 mph.
