Exercise 2. Mr. Deschamps owns chickens and cows; the chickens all have two legs, and the cows have four. In preparation for winter, he must make slippers for them. He has 160 animals in total, and he had to make 400 slippers. How many cows does he have?

Only a numerical answer is expected here.
Solution to exercise 2 Let $p$ be the number of chickens owned by Mr. Deschamps, and $v$ be the number of cows owned by Mr. Deschamps. We know that $p+v=160$ and that $2 p+4 v=400$. We deduce that

$$
v=\frac{(2 p+4 v)-2(p+v)}{2}=\frac{400-2 \times 160}{2}=\frac{80}{2}=40
$$

which means that Mr. Deschamps owns 40 cows.