Each box of milk in Fengxiang Supermarket is 1.4 yuan. If the store has a promotion, buy three and get one free, and buy 20 boxes of milk, how much should you bring at least?
凤祥超市每盒牛奶1.4元，如果商店搞促销活动，买三赠一，买20盒牛奶，至少要带多少钱？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">1.4 * 15</gadget>
<output>21</output>

<result>21</result>