a laboratory technician needs to make a 40-liter batch of a 40% acid solution. how can the laboratory technician combine a batch of an acid solution that is pure acid with another that is 20% to get the desired concentration?

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)