From a flask containing 80 g of a 10% sodium chloride solution, pour some of the solution into a test tube

From a flask containing 80 g of a 10% sodium chloride solution, pour some of the solution into a test tube and evaporate until the salt percentage in the test tube triples. After that, the evaporated solution is poured back into the flask. As a result, the salt content in the flask is increased by 2 percent. Determine how much of the solution was poured from the flask into the test tube?

First, let’s determine how much salt was in the flask initially:

80 * 10% = 80 * 0.1 = 8 (g).

Note that as a result of all actions (transfusion, evaporation) the amount of salt has not changed. Then we determine the mass of the new solution (x) from the proportion:

8 g – (10 + 2)%;

x g – 100%.

Then:

x = 8 * 100%: 12% = 200/3 g.

So, they evaporated:

80 – 200/3 = 240/3 – 200/3 = 40/3 (g).

If there was y g of solution in the test tube, then:

0.1 * y = 0.3 * (y – 40/3);

0.1 * y = 0.3 * y – 4;

0.2 * y = 4 | * five;

y = 20 (d).

Answer: 20 g.