Suppose you work in a lab and you need a 15% acid solution to conduct a certain test, but your supplier only ships a 10% solution and a 30% solution. You need 10 liters of the 15% acid solution?
Let's work this out by saying the amount of 10% solution is
Then the 30% solution will be
The wanted 15% solution contains
The 10% solution will provide
And the 30% solution will provide
You will need 7.5 L of the 10% solution and 2.5 L of the 30%.
You can do this another way. Between 10% and 30% is a difference of 20. You need to go up from 10% to 15%. This is a difference of 5.
So your mix should contain