A waiter had 21 customers in his section. If 12 of them left and the rest of his tables had 3 people at each table, how many tables did he have?
<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<result>3</result>