Question 1122997
x gal of 10% solution
100-x (everything else) of 65% solution
.10x+0.65(100-x)=100*0.35=35
.10x+65-0.65x=35
-.55x=-30
x=54.5 gal of 10% solution 
100-x=45.5 gal of 65% solution

Check--their average is 37.5%, a little high, so this should be fairly even between both, skewed towards the 10% solution, which it is.