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.
