A boat can travel 38 mph in still water. If the boat can travel 100 miles with the current in the same time it can travel 80 miles against the current, find the rate of the current r in miles per hour.

Answer 1

speed of current --- x mph

time to go 100 miles with the current = 100/(38+x)
time to go 80 miles against the current = 80/(38-x)

so 100/(38+x) = 80/(38-x)
cross-multiply
3040 + 80x =3800 - 100x
180x =760
x = 760/180 = 38/9 mph
the current is appr 4.2 mph