Need help with this level four question below.

Question

A chemistry teacher needs to make 10 L of 42% sulphuric acid solution. The acid solutions available are 30% sulphuric acid and 50% sulphuric acid, by volume. How many liters of each solution must be mixed to make the 42% solution? *my algebra teacher told me the answer is 4 L of the 30% solution and 6 L of 50% solutioin, but she didn´t tell me how to get the equations because it is a level four*

Answer 1

Let x = liters of 30% acid

The 10 - x = liters of 50% acid

And the total = 10 liters of 42% acid.

(x liters)(30%) + (10 - x liters)(50%) = (10 liters)(42%)

30x + 500 - 50x = 420

-20x = -80

x = 4 liters of 30%

10-x = 6 liters of 50%