word problem?

Question

chris can row a boat a distance of 20 miles in 2.5 hours when he is rowing with the current it takes him 3 hours to row the same distance rowing against the current.what is the rate of the current??
please explain how you got your answer and write it out if you can <3

Answer 1

C = current's speed
X = Chris's speed

20/2.5 = (X+C) = 8
so X = 8 - C
20/3 = (X-C) = (8 - 2C)
20 = 24 - 6C
C = 2/3
X = 7 1/3

Answer 2

When Chris is rowing with the current, he goes 20/2.5 = 8 miles per hour. When rowing against the correct, he goes 20/3 = 6.66667 miles per hour. The average of these two would be his "normal" rowing speed (7.33333 mph), and the difference between his normal rate either of the other two speeds would be the speed of the current, which is 0.66666667 mph (or 2/3 mph).

Here's another way to look at it:

Chris's speed (C) with the current (X) would be C+X.
Chris's speed (C) against the current (X) would be C-X.
The difference of the two would be (C+X)-(C-X) = 2X
Therefore, if you take half of the difference of the two speeds, you'd get the speed of the current:

2X = (20/2.5) - (20/3) = 8 - (20/3) = (24/3) - (20/3) = 4/3

... and half of 4/3 would be 2/3 mph.