Solution
x + y = 21
x - y = 3
Add both equations to eliminate y
x + y + x - y = 21 + 3
2x = 24
x = 24/2
x = 12
Sunstitute x = 12 into the first equation
12 + y = 21
y = 21 - 12
y = 9
Thus, x = 12, y = 9
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