There are 20 students going on an outing, each with a bottle of mineral water. Buy 4 bottles of mineral water in the supermarket and get 1 free. They only need to buy as many bottles as they want.
有20个同学去郊游，每人带一瓶矿泉水．超市里的矿泉水买4瓶送1瓶，它们只需买多少瓶就可以了．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

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

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>