Question 526664
You have 2 gal. @ 80%, which is .8*2 = 1.6 gal. of pure acid.
.
You need add 'x' gallons of 100% pure acid to make a 90% solution.
.
.8*2 + 1*x = .9(2+x)
.
1.6 + x = 1.8 + .9x
.
multiply by 10 to eliminate decimals
.
16 + 10x = 18 + 9x
x = 2
.
Answer:  Add 2 gallons of pure acid to 2 gallons of 80% acid to make a 90% acid solution.
.
Check the answer:
.8*2 = 1.6 gal of pure acid in the original 2 gal.
1.6 + 2 = 3.6 gal of pure acid in the final solution.
3.6/4 * 100 = 90%.
Correct.
.
Done.