A chemistry teacher needs to mix a 40%

40%

salt solution with a 80%

80%

salt solution to make 20

20

qt of a 70%

70%

salt solution. How many quarts of each solution should the teacher mix to get the desired result?

Let f and e represent the amount of 40% and 80% solutions used ...

We are told the f+e=20 which means that f=20-e and we can say that:

(.4f+.8e) / 20=.7, using f=20-e in this equation we see that:

.4 (20-e) +.8e=14

8-.4e+.8e=14

8+.4e=14

.4e=6

e=15, and since f=20-e then

f=5

So 15 quarts of 80% solution is mixed with 5 quarts of 40% solution to produce a 20 quart mixture of 70% solution.