You need to make 250 ml of a 20% solution. In stock you have 30% solution. How much sterile water do you use for this preparation?

Answer 1

(x ml)(30%) = (250 ml)(20%)

x = 167 mls of the 30% is needed

Thus, the amount of water needed is 250 ml - 167 ml = 83 ml

You would take 167 ml of the stock 30% solution and add 83 ml of sterile water.