In still water the speed of boat is 10 mph.Against current it travels 4 miles in the same amount of time it can travel 6 miles with the current. What is speed of current?

Question

The answer in the back of the book is 2mph. I just can't figure out how to solve it and put it in an equation. Thanks in advance for the help!

Answer 1

B=speed of boat=10mph; C=speed of current.

Effective speed against current is B-C=10-C.

Effective speed with current is B+C=10+C.

Distance=speed*time, so time=distance/speed.

Time against current=4/(10-C)=time with current=6/(10+C).

Cross-multiply: 4(10+C)=6(10-C), 40+4C=60-6C, 10C=20 so C=2mph.