Question 877598
"4 quarts of a 40% antifreeze solution" ---> 4*0.40 = 1.6 quarts of pure antifreeze.


Adding some unknown amount, call it x, of pure antifreeze gives you 1.6+x quarts of pure antifreeze.


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You have 4 quarts of a mixed solution and you're adding x quarts of pure antifreeze to the mix. 
So you end up with a total of 4+x quarts of mixed solution.


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(amount of pure antifreeze)/(amount of solution) = percent of antifreeze



(1.6 + x)/(4 + x) = 0.60



1.6 + x = 0.60(4 + x)



1.6 + x = 0.60(4) + 0.60(x)



1.6 + x = 2.4 + 0.60x



1.6 + x - 0.60x = 2.4



x - 0.60x = 2.4 - 1.6



0.40x = 0.8



x = 0.8/0.40



x = 2



You need to add <font color="red">2 quarts</font> of pure antifreeze.