Question 146201
Let x=amount of pure antifreeze that needs to be added

Now we know that the amount of pure antifreeze in the quantity that was added (x) plus the amount of pure antifreeze in the 300 gal (0.30*300) has to equal the amount of pure antifreeze in the final mixture(0.40(300+x)).  So our equation to solve is:

x+0.30*300=0.40(300+x) get rid of parens (distributive law)
x+90=120+0.40x subtract 0.40x and also 90 from each side
x+90-90-0.40x=120-90+0.40x-0.40x  collect like terms

0.60x=30  divide each side by 0.60
x=50 gal-------------------------------amount of pure antifreeze needed

CK

50+90=0.40(350)
140=140


Hope this helps------ptaylor