Each basketball is 100 yuan, shopping mall promotion, buy 4 get 1 free, buy 4 at a time, how much is each cheaper.
每个篮球100元，商场促销，买4送1，一次购买4个，每个便宜多少元．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">100 / 5</gadget>
<output>20</output>

<gadget id="calculator">4 * 20</gadget>
<output>80</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<result>20</result>