how many grams of pure water must be added to 40 grams of a 10% saline aokution to make a saline solution tha is 5% salt?

40

Step-by-step explanation:

1) the basic is: the mass of the salt before pure water is added = the mass of the salt after the pure water is added;

2) the mass of the salt before pure water is added was:

40*0.1;

3) the mass of the salt after pure water is added is:

(40+w)*0.05, where w - the required mass of pure water;

4) if the mass of the salt is the same, then:

40*0.1=(40+w)*0.05;

5) if to evaluate the equation:

(w+40)*0.05=4; ⇔ w+40=80; ⇔ w=40 gr.