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First factor the each term in the expression.

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15 3 · 5.

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20x 2 · 2 · 5 · x

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The GCF is the product of the common prime factors in each term.

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GCF = 5

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Write each term as a product of the GCF and its remaining factors.

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15 + 20x = 5 (3) + 5 (4x)

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Combine like terms uisng distribuitve property: ab + ac = a(b + c)

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15 + 20x  = 5 (3 + 4x)\"\"

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Simplified expression is 5(3 + 4x).