Question 830185
Let A denote the 70% solution and B denote the 90% solution

Using the method of alligation,

0.75-0.70=0.05

0.90-0.75=0.15

So that A/B=0.15/0.05

E.g. there are 0.15 parts of A for every 0.05 parts of B

If there are 100 parts of A, then (0.15/0.05)=100/x

x=33.3 gallons of 90% solution (Answer)