Question 180882
You need to apply the distance formula:
d = rt
where
d is distance
r is rate or speed
t is time
.
Let x = speed of the current
then
12/(15+x) = 9/(15-x)
12(15-x) = 9(15+x)
4(15-x) = 3(15+x)
60-4x = 45+3x
60 = 45+7x
15 = 7x
15/7 = x
or
2.1429 mph = x