Problem 1. In a store, there are 800 notebooks and it operates from Monday to Friday. In one week, all the notebooks were sold such that each day 40 more notebooks were sold than the previous day. How many notebooks were sold each day of that week?
Solution. Let $x$ be the number of notebooks sold on Monday. Then, on Tuesday, $x+40$ notebooks were sold, on Wednesday $x+80$, on Thursday $x+120$, and on Friday $x+160$ notebooks. All notebooks were sold, so

$$
x+x+40+x+80+x+120+x+160=800
$$

From this, it follows that $5 x+400=800$, i.e., $x=80$. Therefore, 80 notebooks were sold on Monday, 120 on Tuesday, 160 on Wednesday, 200 on Thursday, and 240 on Friday.