There are 4 boxes of balls in the store, each with 6 balls. 20 sold, how many left?
商店里有4盒皮球，每盒6个．卖出20个，还剩多少个？
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<result>4</result>