Answer:
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.
