Let the speed of the current be x mph.
With the current, the effective speed of the boat is 51 + x mph.
Against the current, the effective speed of the boat is 51 - x mph.
Let t be the time taken to travel 424 miles with the current and 392 miles against the current.
Using the formula Distance = Speed x Time:
424 = (51 + x)t
392 = (51 - x)t
We now have a system of equations to solve:
51t + xt = 424
51t - xt = 392
Adding the two equations together, we get:
102t = 816
t = 8 hours
Substitute t=8 back into one of the original equations:
424 = (51 + x) * 8
424 = 408 + 8x
16 = 8x
x = 2
2 mph.
