Question 197307
Problem  #1
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(1) 2x +2y = 140
(2) x-y =20 
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Your  start  was  good,,(1)  becomes  y=-x+70,,,and  (2)  becomes  y = x-20
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Setting  these  equal  to  each  other,  substitution,
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-x+70 = x-20
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90 = 2x
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x=45,,,answer
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Subst  into  (2) ,,,45 -y = 20,,,y=45-20,,,,y = 25
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checking
in (1)  2(45) + 2(25) = 140,,,140 =140,,,ok
in  (2)  (45) - (25) = 20,,,20 = 20,,,ok