5. In the village where Glafira lives, there is a small pond that is filled by springs at the bottom. Curious Glafira found out that a herd of 17 cows completely drank the pond dry in 3 days. After some time, the springs refilled the pond, and then 2 cows drank it dry in 30 days. How many days would it take for one cow to drink the pond dry?
5. Answer: In 75 days.

Solution. Let the pond have a volume of a (conditional units), one cow drinks b (conditional units) per day, and the springs add c (conditional units) of water per day. Then the first condition of the problem is equivalent to the equation $a+3c=3 \cdot 17 b$, and the second to the equation $a+30c=30 \cdot 2 b$. From this, we get that $b=3c, a=150c$. If one cow drinks the pond dry in x days, then we have $a+xc=xb$, that is, $x=\frac{a}{b-c}=75$.