SOLUTION: A rectangular lawn has dimensions 30m by 40m. A strip of uniform width is to be placed around the perimeter. How wide should the strip be so that exactly half the lawn gets mowed?

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Question 118528: A rectangular lawn has dimensions 30m by 40m. A strip of uniform width is to be placed around the perimeter. How wide should the strip be so that exactly half the lawn gets mowed?
I tried (2x-30)(2x-40) and found x=5m but I was marked wrong by my teacher :(
Answer by scott8148(6628) About Me  (Show Source):
You can put this solution on YOUR website!
5m is correct, but what was your technique?

the lawn is 30 by 40 or 1200 sq m, so half would be 600 sq m

(30-2x)(40-2x)=600 ___ 1200-140x+4x^2=600 ___ 600-140x+4x^2=0

x^2-35x+150=0 ___ (x-30)(x-5)=0