Find the speed of the current.

Question

In still water a riverboat can travel 12 miles per hour. It travels 48 miles upstream and then 48 miles downstream in a total time of 9 hours. Find the speed of the current.

Answer 1

48/(12 + s) + 48/(12 - s) = 9

48(12 - s) + 48(12 + s) = 9(12 - s)(12 + s)

576 - 48s + 576 + 48s = 1296 - 9s²

9s² - 144 = 0

(3s + 12)(3s - 12) = 0

 

3s + 12 = 0

3s = -12

s = -4

No negative measurements!

 

3s - 12 = 0

3s = 12

s = 4

 

The speed of the current is 4 mph.