Example 4 A store sells goods that cost 10 yuan each at 18 yuan each, and can sell 60 of them per day. After conducting a market survey, the store manager found that if the selling price of the goods (based on 18 yuan each) is increased by 1 yuan, the daily sales volume will decrease by 5; if the selling price of the goods (based on 18 yuan each) is decreased by 1 yuan, the daily sales volume will increase by 10. To maximize daily profit, what should the selling price of the goods be set at per unit?
(Tenth Zǔ Chōngzhī Cup Junior High School Mathematics Invitational Competition)
Solution: Let the selling price of each item be $x$ yuan, and the daily profit be $s$ yuan.
$$
\begin{array}{l}
\text { When } x \geqslant 18 \text {, we have } \\
s=[60-5(x-18)](x-10) \\
=-5(x-20)^{2}+500,
\end{array}
$$

That is, when the selling price of the item is increased to $x=20$ yuan, the daily profit $s$ is maximized, with the maximum daily profit being 500 yuan.
$$
\begin{array}{l}
\text { When } x \leqslant 18 \text {, we have } \\
s=[60+10(18-x)](x-10) \\
=-10(x-17)^{2}+490, \\
\end{array}
$$

That is, when the selling price of the item is reduced to $x=17$ yuan, the daily profit is maximized, with the maximum daily profit being 490 yuan.
In summary, the selling price of this item should be set at 20 yuan each.