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The equation is 18 (f) = 91.

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18 + f = 91                             (Product of two same signs is positive)

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According to subtraction property of equality: If a = b, then a c = b c.

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18 + f  18 = 91 18      (Subtract 18 from each sides)

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  f = 91 18                         (Additive inverse property: 18 18 = 0)

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  f = 73                                  (Subtract:  91 18 = 73)

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Check:

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To check the solution, substitute f = 73 in original equation.  

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 18 + f = 9118 + 73 = 91        

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 91 = 91                                  (Add: 18 + 73 = 91)

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Compare the values, the equation is true. The solution is f = 73.

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The value of f = 73.

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