needs 200 ml of a solution that is 7% NaCl. Have solutions that are 15% NaCl and 5% NaCl.How many ml of each solution should be mixed to get 200 ml of 7% NaCl?

Question

For an experiment, a student requires 200 milliliters of a solution that is 7 % NaCl (sodium chloride). The storeroom has only solutions that are 15 % NaCl and 5 % NaCl. How many milliliters of each available solution should be mixed to get 200 milliliters of 7 % NaCl?

How many ml of the 5% solution should be used? 

How many ml of the 15% solution should be used?

Answer 1

15x + (200-x)5 = 200(7)

15x-5x + 200(5) = 200(7)

10x = 400

x = 40 ml of 15%

200-40 = 160 ml of 5%

5 is within 2 of 7

15 is within 8 of 7

5 is 4 times closer to 7

so you need 4 time more of 15% than 5%

160 is 4 times 40 ml