A certain burger in the supermarket is 10 yuan each. This burger has recently launched a "buy two get one free" promotion, that is, if you spend money to buy two burgers, you can get a free burger. It is known that Dongdong needs to buy 9 hamburgers, so how much is the least they need to spend.
超市中的某种汉堡每个10元，这种汉堡最近推出了"买二送一"的优惠活动，即花钱买两个汉堡，就可以免费送一个汉堡．已知东东需要买9个汉堡，那么他们最少需要花多少元钱．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">10 * 6</gadget>
<output>60</output>

<result>60</result>