4.) A pharmacist has to fill a prescription calling for 5 ounces of a 12% argyrol solution. She has 5 ounces of a 15% solution and 5 ounces of a 5% solution. If she starts with the 5 ounces of 15%, how much must she draw off and replace with 5% in order to fill the prescription?

Ounces of draw off and replace in 15% solution (x):
0.15( 5 - x) + 0.05x = 0.12(5)
3(5 - x) + x = 12/5(5)
15 - 3x + x = 12
2x = 3
x = 3/2 or 1 1/2
1 1/2 ounces draw off and replace
Proof (12% solution in 5 ounces after draw off & replace):
= 100%([0.15{5 - 1 1/2 oz} + 0.05{1 1/2 oz}]/5 oz)
= 100%([0.15{3 1/2 oz} + 0.05{3/2 oz}]/5 oz)
= 100%(0.15{7/2 oz} + 0.075 oz]/5 oz)
= 100%([0.525 oz + 0.075 oz]/5 oz)
= 100%(0.6 oz/5 oz)
= 100%(0.12)
= 12%

Ithink it should be 55.