$\"\"$

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The given values $\"\"$ .

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Where $\"\"$ is the first term and $\"\"$ is the common difference.

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The sum of an arithmetic series $\"\"$ .

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$\"\"$ (Substitute $\"\"$ )

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

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$\"\"$ (Multiply: $\"\"$ ) $\"\"$

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Apply multiplication property of equality: a = b then $\"\"$ .

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$\"\"$ (Multiply each side by 2)

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$\"\"$ (Cancel common terms)

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Apply division property of equality: a = b then $\"\"$ .

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$\"\"$ (Divide each side by 31)

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$\"\"$ (Cancel common terms)

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

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Apply subtraction property of equality: a = b then $\"\"$ .

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$\"\"$ (Subtract 15 from each side)

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$\"\"$ (Apply additive inverse property: $\"\"$ )

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$\"\"$ (Apply additive addition property: $\"\"$ )

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

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Apply division property of equality: a = b then $\"\"$ .

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$\"\"$ (Divide each side by 2)

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$\"\"$ (Cancel common terms)

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

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The value of $\"\"$ .