Old Beijing yogurt is 4 yuan per box. A supermarket recently launched a "buy two get one free" promotion. That is, if you buy two boxes of yogurt, you can get a lot of yogurt for free. Beibei wants to buy 10 boxes of old Beijing yogurt. So how much money will he spend at least.
老北京酸奶每盒4元．某超市最近推出"买二送一"的优惠活动．即买两盒酸奶就可以免费获得一众酸奶．北北要买10盒老北京酸奶．那么他至少要花多少元钱．
<gadget id="calculator">3 - 3</gadget>
<output>0</output>

<gadget id="calculator">7 + 0</gadget>
<output>7</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>