A+17=(A-13)2
A+17=2A-26
A=43
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two equation:
1.D-B=36
2.(D+20+B)/2=42
put 1. into 2.
(D+20+D-36)/2=42
2D-16=84
D=50
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two equations again
1.72=X+Y
2.X+Y=2(X-Y)
2. can be simiplifed as:
X+Y=2X-2Y
X=3Y put it back to equation 1.
72=4Y
Y=18
X=54