How much of each solution should be used if 21 gallons of 4% solution are needed?

Question

A mixture of 4% disinfectant solution is to be made from 5% and 2% disinfectant solutions. How much of each solution should be used if 21 gallons of 4% solution are needed?

Answer 1

Let x = number of gal of 2% solution

 

So, 21 - x = number of gal of 5% solution

 

0.02x + 0.05(21 - x) = 0.04(21)

 

0.02x + 1.05 - 0.05x = 0.84

 

                     -0.03x = -0.21

 

                             x = 7

 

Use 7 gal of 2% solution and 14 gal of 5% solution to obtain 21 gal of 4% solution.