Convert your dilution problem into statements of proportions. For example, you might have 2.5L of a 30% solution of hydrochloric acid that you have to convert into a 9% solution, and you have to decide how much water to use. On the left side, right 750 mL / 2500 mL (the 30% solution). Then, write an equals sign, and then write 9/100 (the desired proportion). So, 750 mL/2500 mL = 9/100.
Multiply the 750 mL by 100 to get 75,000, and then divide by 9 to get 8,333.33 mL.
Subtract the initial volume (2,500 mL) from the answer in Step 2. This is the amount of water you'll need to add to dilute to a the desired solution. In this case, the amount would be 5,833.33 mL.