Gave needs 60 liters of 40% acid solution he currently has 20% solution and 50% solution how many liters of each does he need to make the needed 60 liters of 40% acid solution

notice above...  we use the decimal format for the percentage, thus 20% is really just 20/100, 40% is 40/100 and so on.

so.. whatever "x" and "y" amounts are, we know they have to add  up to 60 Liters, that is x+y = 60

and whatever the concentration amount of each is, it must add up to (60)(0.40), that is 0.20x+0.50y=(60)(0.40)

thus    

solve for "x", to see how much of the 20% solution will be needed

what about "y"?  well, y = 60 - x