The school organizes field trips, each person prepares a bottle of mineral water, and the unit price is 2 yuan. You can only buy the whole box, 10% off for a small box of 10 bottles, and 20% off for a large box of 20 bottles. There are 280 students in the whole grade. Please think of the most economical purchase method and calculate the amount of money needed.
学校组织到野外旅游，每人备1瓶矿泉水，单价是2元．只能整箱买，小箱10瓶打9折，大箱20瓶打8折．全年级有280人．请想一种最省钱的购买方法，并计算所需钱数．
<gadget id="calculator">280 / 20</gadget>
<output>14</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * 14 * 2 * (4/5)</gadget>
<output>448</output>

<result>448</result>