​Connie's boat travels at 4 mph. Find the rate of the current of the river if she can travel 3 miles upstream in the same time she can travel 21 miles downstream.

Answer 1

distance = rate * time

3 = (4-x)*t

21 = (4+x)*t

3/(4-x) = 21/(4+x)

3(4+x) = 21(4-x)

12 + 3x = 84 - 21x

3x = 72 - 21x

24x = 72

x=3

The speed of the current is 3