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$\"\"$

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The equations are

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$\"\"$ Equation (1)

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$\"\"$ Equation (2)

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Solve the equation 2 for variable c in terms of the variable d.

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$\"\"$

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$\"\"$ (Subtract 3d from each side)

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

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Substitute $\"\"$ for c in equation 1.

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$\"\"$

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$\"\"$ (Apply distributive property)

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$\"\"$ (Simplify)

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

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$\"\"$ (Subtract 40 from each side)

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$\"\"$ (Simplify)

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$\"\"$ (Divide each side by -10)

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

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Substitute $\"\"$ for d in $\"\"$ .

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$\"\"$

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$\"\"$ (Simplify)

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

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The solution is $\"\"$ .

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