Each box of candy in the store is 20 yuan, and the special price is to buy 4 boxes and get 1 box free. How much is it cheaper to buy 4 boxes at a time?
商店里每盒糖果20元，特价买4盒送1盒．一次买4盒，每盒便宜多少钱？
<gadget id="calculator">1 + 4</gadget>
<output>5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 * (1/5)</gadget>
<output>4</output>

<result>4</result>