Apharmacist has an 18% alcohol solution and a 40% alcohol solution. how much of each should he mix together to make 10 liters of a 20% alcohol solution?

The amount of alcohol is unchanged after adding water. So, if there are x liters of 18% solution, you have 

.18x = .12*10 
x = 20/3 L of 18% alcohol, and 10/3 L of water 

Or, you can consider that you are reducing the concentration by a factor of 2/3, so you must increase the volume by a factor of 3/2. So, the volume v of 18% alcohol can be found by 

3/2 v = 10 
v = 20/3