Question 530789
Let x=amount of pure antifreeze needed
Then 14-x=amount of 50% antifreeze needed

Now we know that the amount of pure antifreeze that exists before the mixture takes place has to equal the amount of pure antifreeze that exists after the mixture takes place, sooooo:
x+0.50(14-x)=0.70*14
x+7-0.50x=9.8 simplify
0.50x=2.8
x=5.6 quarts----amount of pure antifreeze needed

CK
5.6+0.50*8.4=0.70*14
5.6+4.2=9.8
9.8=9.8

Hope this helps----ptaylor