18. (10 points) There are three pastures full of grass. The first pasture is 33 acres and can feed 22 cows for 27 days; the second pasture is 28 acres and can feed 17 cows for 42 days; the third pasture is 10 acres and can feed how many cows for 3 days (assuming the amount of grass per acre is the same for each pasture, and it grows at a uniform rate)?
【Solution】Solution: Each cow eats 1 unit of grass per day, each mu (Chinese unit of area) originally has $x$ units of grass, and each mu grows $y$ units of grass per day,
$$
\begin{array}{l}
27 \times(22-33 y)=33 x, \text { (1) } \\
42 \times(17-28 y)=28 x, \text { (2) }
\end{array}
$$

Solving equations (1) and (2) simultaneously, we get: $y=0.5, x=4.5$
Then, $(10 \times 4.5+0.5 \times 10 \times 3) \div 3$,
$$
\begin{array}{l}
=60 \div 3, \\
=20 \text { (cows); }
\end{array}
$$

Answer: The third pasture, 10 mu, can support 20 cows for 3 days.