Concentration with x preparation volume = desired concentration x total volume after preparation. Let's say we use
แอลกอฮอล์ 95% 100 ml volume of the preparation as
แอลกอฮอล์ 70% we can substitute into the equation 95 x 100 = 70 / total volume after preparation which will get the total volume after preparation is about 135.7, indicating that we need to add more water to the
แอลกอฮอล์ 95% 100 ml volume, about 35 ml more to get
แอลกอฮอล์ 70%