mixture problems

Question

You need 30% alcohol solution. On hand you have 240 ML of a 35% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

You will need _____ mL of pure water to obtain ___ mL of the desired 30% solution

Answer 1

Let Y=30% solution quantity, X=35% solution, W=water quantity

240(.35) + W(0) = Y(.30)

84 = Y(.30)

Y=84/.3 = 280 ml

W=280-240 = 40 ml