algebra
posted by rf .
how many mls of pure water does a pharmacist have to add to 50mls of a 15% minoxide solution to get a 10% minoxide solution?

The total solute must add up. Pure water is 0% solution:
w(0) + 50(.15) = (w+50)(.10)
7.5 = .10w + 5
.10w = 2.5
w = 25 ml
Just to check. We added 50% more volume. The volume is 3/2 what it was, so the concentration is 2/3 what it was.