9. (5 points) In a charity sale, Wang Gang sold a total of 400 cups of lemonade and hot chocolate, earning 546 yuan. If lemonade is 1 yuan per cup and hot chocolate is 2 yuan per cup, then how many cups of lemonade did Wang Gang sell in this charity sale? $\qquad$ cups of lemonade.
【Analysis】Assume all 400 cups are hot chocolate, then the total amount would be $400 \times 2=800$ dollars, which is 800 $-546=254$ dollars more than the known 546 dollars. Since a cup of hot chocolate is 2-1=1 dollar more expensive than a cup of lemonade, it follows that there are 254 cups of lemonade, and this can be used to solve the problem.

【Solution】Solution: $(400 \times 2-546) \div(2-1)$,
$$
\begin{array}{l}
=254 \div 1, \\
=254 \text { (cups), }
\end{array}
$$

Answer: Wang Gang sold 254 cups of lemonade in this charity sale.
The answer is: 254.
【Comment】This problem belongs to the category of the "chicken and rabbit in the same cage" problem, and can be solved using the assumption method.