We have #25%# solution and a #50%# solution of a chemical. How do we mix them to prepare #100# gallons of #40%# solution?
The easiest way to solve questions relating to such mixtures is depicted in following diagram.
First write original strength as indicated in first row.
Then write desired strength in the center.
Then find the difference, using the cross.
This gives the ratio, in which original items should be mixed to get desired strength.
And as we desire