2. (8 points) A certain hamburger in the supermarket costs 10 yuan each. This hamburger has recently launched a "buy two get one free" promotion, which means if you buy two hamburgers, you can get one hamburger for free. It is known that Dongdong and his friends need to buy 9 hamburgers. How much money do they need to spend at least? $\qquad$ yuan.
2. (8 points) A certain hamburger in the supermarket costs 10 yuan each. This hamburger has recently launched a "buy two get one free" promotion, which means buying the first two hamburgers will get you one hamburger for free. It is known that Dongdong and his friends need to buy 9 hamburgers, so they need to spend at least 60 yuan.
【Solution】Solution: $9 \div(2+1)=3$ (units)
$$
\begin{array}{l}
10 \times[9 \div(2+1) \times 2] \\
=10 \times[9 \div 3 \times 2] \\
=10 \times 6 \\
=60 \text { (yuan); }
\end{array}
$$

Answer: They need to spend at least 60 yuan.
Therefore, the answer is: 60.