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Substitution method : 

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Given equations are

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  ---> (1)

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x + y  = 18       ---> (2) \ \

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solve for y from (2).

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Subtract y from each side.

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x = 18 - y         ---> (3)

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Substitue x = 18 - y in (1). \ \

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Substitute in (3)

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x = 18 - 12 = 6.

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Solution  x = 6 and y = 12.

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