Aboat makes a 120-mile trip downstream in 3 hours but makes the return trip in 4 hours.. what is the rate of the current? . a) 3 mph. b) 4 mph. c) 5 mph.

Let the still water boat speed be b, and let the rate of the current be c.
3(b + c) = 120 (1)
4(b - c) = 120 (2)
Multiply (1) by 4 and multiply (2) by 3, to get:
12b + 12c = 480 (3)
12b - 12c = 360 (4)
Add equations (3) and (4):
24b = 840
b = 35 mph
Plugging the value of b into equation (3) and solving for c, gives the value of 5 for c.
Therefore answer choice C) 5 mph is correct.

Option c is correct answer because 
Let c = rate of current 
Let b = rate of boat in still water 
Then b+c = rate downstream 
b-c = rate upstream 
So 3(b+c) = 120 = b+c = 40 
And 4(b-c) = 120 = b-c = 30 
So 2b = 70 ... b = 35 mph speed in still water 
So c = 5 mph = current speed

The answer would be (c)