Question 1200059
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Let V be the total volume of the mixture (antifreeze + water).


Then 0.85V should be 1 gallon of the pure antifreeze

    0.85V = 1  gallon.


From this equation,  V = {{{1/0.85}}} = 1.176 gallon (rounded).


It means that the volume  1.176 - 1 = 0.176 gallon of water should be added.      <U>ANSWER</U>
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Solved.