A boat can go 20 miles against a current in the same times that it can go 60 miles with the current. The current is 4 miles per hour. Find the speed of the boat with no current.

Answer 1

d1 = (Vb-4)t = 20
Eq1: Vb*t - 4t = 20

d2 = (Vb+4)t = 60
Eq2: Vb*t + 4t = 60

Multiply Eq1 by -1:
-Vb*t + 4t = -20
+Vb*t + 4t = 60
Sum:0 + 8t = 40
t = 5 h.

In Eq1, replace t with 5:
Vb*5 - 4*5 = 20
5Vb = 40
Vb = 8 mi/h = Velocity of the boat.