Question

The speed of the current is 7 mph. The boat travels three times slower against the current than with the current. What is the speed of the boat in still water?

Answer 1

Let the speed of the boat be x
so going against the current the speed is x-7
going with the current the speed is x+7

let the distance be d
time = distance / speed
time to go against the current = d/(x-7)
time to go with the current = d/(x+7)

time going against the current is 3 times going with the current

d/(x-7) = 3d/(x+7)     ⇒⇒⇒ divide by d 
1/(x-7) = 3/(x+7) 
x+7 = 3(x-7) 
x+7 = 3x - 21
2x = 28
x = 14

So, the speed of the boat in still water is = 14 mph