Answer :
the answers comes to be for every 1 unit of pure acid (100%) add three units of 20% and you get the desired 40%
so if you need 40 L (i don't know what container sizes there are so i can't give you an exact amount just add 1 part 100% acid and 3 parts 20% acid)
so if you need 40 L (i don't know what container sizes there are so i can't give you an exact amount just add 1 part 100% acid and 3 parts 20% acid)