139. On a meadow, grass grows. If 9 cows were let onto the meadow, they would empty it in 4 days. If 8 cows were let onto the meadow, they would eat all the grass in 6 days. How many cows can graze on the meadow all the time while the grass grows?
Solution. Let $x$ kilograms of grass grow on the meadow per day, and one cow eats $y$ kilograms of grass per day. We have the system of equations:

$$
\left\{\begin{array}{l}
a+4 x-9 \cdot 4 y=0 \\
a+6 x-8 \cdot 6 y=0
\end{array}\right.
$$

where $a$ is the amount of grass that grew on the meadow before the cows were let in. Subtracting the first equation from the second, we find $x=6 y$. If the desired number of cows is $n$, then $n y=x$, from which $n=\frac{x}{y}=6$.

Remark. This problem can also be solved without setting up a system of equations. Indeed, if $a$ kilograms is the initial amount of grass, and $x$ kilograms is the amount of grass that grows on the meadow per day, then in one day one cow eats $\frac{a+4 x}{36}$ or $\frac{a+8 x}{48}$ kilograms of grass. We have the equation

$$
\frac{a+4 x}{36}=\frac{a+8 x}{48},
$$

from which $a=2 x$. Therefore, one cow eats $\frac{x}{6}$ kilograms of grass per day, where $x$ kilograms is the amount of grass that grows on the meadow per day. Therefore, 6 cows can be fed on the meadow.

Answer: 6 cows.