20 gallons of 80% = 20 (.8) = 16 gal of pure ethanol needed
x = amount of 90% needed
20 gal - x = amount of 65% needed
x(.9) + (20-x)(.65) = 16
.9x + 13 - .65x = 16
.25x = 3
x= 12 gallons of 90% needed so 20-12 = 8 gallons of 65% needed
12-8 = = 4 gallons MORE of the 90% than the 65% needed
