Question

A motorboat can go 8 miles downstream on a river in 20 minutes. It takes 30 minutes for the boat to go upstream the same 8 miles. Find the speed of the current.

Answer 1

Answer:

4 mph

Step-by-step explanation:

Let's say m is the speed of the motor boat, and r is the speed of the river.

20 minutes is 1/3 hour, and 30 minutes is 1/2 hour.

Distance = rate × time

8 = (m + r) × 1/3

8 = (m − r) × 1/2

We want to find the value of r.  Let's start by solving for m in one equation, then substitute into the other.

8 = (m − r) × 1/2

16 = m − r

m = r + 16

8 = (m + r) × 1/3

8 = (r + 16 + r) × 1/3

8 = (2r + 16) × 1/3

24 = 2r + 16

8 = 2r

r = 4

The speed of the river's current is 4 miles per hour.