You need a 50% alcohol solution. On hand, you have a 200 mL of a 20% alcohol mixture.

Question

How much pure alcohol will you need to add to obtain the desired solution?

Answer 1

Use a weighted average: Let x = the amount of 100% alcohol to be added

Then, { 200(20%) + x(100%) }/(200 + x) = 50% The % will cancel so,

200(20) + 100x = 50(200 + x) or 4,000 + 100x = 10,000 + 50x

So, 50x = 6,000 and x = amount of 100% alcohol = 120 ml