3. 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?
Answer: In 75 days.

Solution. Let the pond have a volume of $a$ (conditional units). These units can be liters, buckets, cubic meters, etc. Let one cow drink $b$ (conditional units) of water 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 $a+3c=3 \cdot 17b$, and the second to the equation $a+30c=30 \cdot 2b$. We have obtained a system of two equations with three unknowns. The unknowns cannot be found from here, but the relationship between them can be established.

Subtracting the first equation from the second, we get $b=3c$. Substituting into one of the equations gives $a=150c$.

If one cow drinks the pond dry in $x$ days, then we get $a+xc=xb$, that is, $x=\frac{a}{b-c}=75$ days.

Answer to variant 172: In 140 days.

Answer to variant 173: In 80 days.

Answer to variant 174: In 150 days.