Grigori S. answered • 04/18/13

Certified Physics and Math Teacher G.S.

Let x be the speed of tugboat without a current and y be the speed of the current.

Upstream speed is "x-y" (because of the stream resistance) and the downstrean speed is "x+y" (the stream helps to move faster). According to the text

20(x-y) = 160

10(x+y) = 160

Simplify the system of these two equations dividing both sides of the firsdt equation by 20 and the second one by 10. We will obtain

x-y = 8

x + y = 16

Addition of two equations gives 2x = 24 or x =12. Consequently y = 12 - 8 = 4 or 16-12 =4.

Summarize: x= 12 y = 4.