Question 1007386
2L+2W= Perimeter
2L+2W= 140
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L= 2W+10 Let's use this value for L:
2L+2W= 140
2(2W+10)+2W= 140 Multiply on left to remove parenthesis 
4W+20+2W= 140 Add W on left and subtract 20 on both sides
6W= 120 Divide both sides by 6
W= 20 This is the width, and the problem says that the length is:
L= 2(20)+10= 50
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Check:
2(50)+2(20)= 140
100+40= 140
140= 140 We have the correct answer