Question 1031938
I just answered this question, I don't know if you posted it twice or if someone else had the same question: 
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r= rate of child on a still sidewalk
w= walkway speed
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formula: distance = rate*time
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13(r-w) = 39
13(r+w) = 65
Divide all by 13 in order to simplify the equations:
r-w = 3
r+w = 5 Add both equations to eliminate one variable (-w cancels +w):
2r = 8
r = 4 This is the speed of the child. Now substitute to find w:
13(4-w) = 39
52-13w = 39
-13r = -13
w = 1 This is the speed of the sidewalk.
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With the sidewalk:
4+1=5; 5*13= 65 correct
Against the sidewalk:
4-1=3; 3*13=39 also correct