What happens when you have more than 2 sources of the component?
For example in the following:
"A pharmacist needs to prepare a 25% Dextrose solution from a 50% solution, a 15% solution and a 5% solution. How much of each solution should be used to prepare 560ml of 25% solution?"
For this problem, You pair up the source which is greater than the required amount with the sources which are less than the required amount ie you pair up:
50% Dextrose with 15% Dextrose
and
50% Dextrose with 5% Dextrose.
Then you find out the relative proportions of EACH of the pairs needed to produce the 25% solution.
You can think of the method in the following way:
you are "preparing" on paper TWO 25% solutions: the first solution is made using the 50% and the 15% solution, the second is made using the 50% and 5% solution. The two 25% solution are then mixed together.