story problem

Question

A pharmacist is to prepare 15 milliliters of special eye drops for a glaucoma patient. The eye-drop solution must have a 4% active ingredient, but the pharmacist only has 10% solution and 1% solution in stock. How much of each type of solution should be used to fill the prescription?

Answer 1

x is the amount of the 10% solution
15 - x is the amount of the 1% solution.

(15ml)(.04) = (xml)(0.10) + (15-x)ml (0.01)
since ml is common to both sides, we can factor the units out for now, but we will want to remember that the units of the answer will be in milliliters.

0.6 = 0.1x + 0.15 - 0.01x

combine like terms and isolate the variable
0.6 - 0.15 = 0.9x + 0.15 - 0.15
0.45 = 0.9x
5 = x    5ml of the 10% solution

15 -x = 15 - (5) = 10    10 ml of the 1% solution