$\"\"$ $\"\"$ $\"\"$ $\"\"$

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According to Means-extremes Property of proportion: If $\"\"$ , then $\"\"$

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$\"\"$ (Cross products)

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$\"\"$ (Distributive property: $\"\"$ ) $\"\"$ (Distributive property: $\"\"$ ) $\"\"$

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According to addition property of equality: Add 40 to each side.

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$\"\"$

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$\"\"$ (Additive inverse property: $\"\"$ )

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$\"\"$ (Additive identity property: $\"\"$ )

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$\"\"$ (Add: $\"\"$ ) $\"\"$

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According to subtraction property of equality: Subtract 15n from each side.

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$\"\"$

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$\"\"$ (Additive inverse property: $\"\"$ )

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$\"\"$ (Additive identity property: $\"\"$ )

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$\"\"$ (Subtract: $\"\"$ ) $\"\"$

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$\"\"$ (Divide each side by 5)

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$\"\"$ (Cancel common terms)

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$\"\"$ (Divide: $\"\"$ ) $\"\"$

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The value of $\"\"$ .