10. (6 points) Xiaoming went to the store to buy a total of 66 red and black pens. The red pens are priced at 5 yuan each, and the black pens are priced at 9 yuan each. Due to the large quantity purchased, the store offered a discount, with red pens paid at 85% of the listed price and black pens paid at 80% of the listed price. If the amount he paid was 18% less than the listed price, then how many red pens did he buy? $\qquad$ .
【Solution】Solution: Let the number of red pens be $x$, then the number of black pens is $66-x$,
The original price for buying pens is: $5 x+9 \times(66-x)$,
The price after discount is: $5 \times 85 \% \times x+9 \times 80 \% \times(66-x)$
From this, we can get: $[5 x+9 \times(66-x)] \times(1-18 \%)=5 \times 85 \% \times x+9 \times 80 \% \times(66-x)$
Solving for $x$ yields
$$
x=36 \text {; }
$$

Answer: He bought 36 red pens.