Answer:
q = 0.5 quarts of 100% antifreeze
Step-by-step explanation:
q = quarts of pure antifreeze
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Set this up as a weighted combination of the mixtures.
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(100%)(q) + (10%)(4) = (20%)(q + 4)
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100q + 40 = 20(q + 4)
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5q + 2 = q + 4
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4q = 2
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q = 0.5 quarts of 100% antifreeze