Question 747109
Because this is a relatively simple one, we can just use a little logic to solve it.

 A 40% solution is directly in the middle of the 30% and 50% solution, so equal amounts of each will be used, so just divide 120(total liters needed)/2(number of solutions available)

120/2=60

You will need 60 liters of the 30% and 60 liters of the 50% solution to equal 120 liters of 40% solution.

Harder problems will require more in depth arithmetic, but for this problem, this is enough.