How much pure acid should be mixed with 6 gallons of a 60% acid solution in order to get an 80% acid solution?

So pure=100%

x added

100%=1=pure

amount times purity=result

x=amount added

x times 1

6-x times (0.6) for te 60%

they add up to

6 times 0.8

solve

x + (6-x) 0.6=6 (0.8)

x=3

answer is 3 gallons replaced with 100% acid