Xinhua Bookstore shipped 540 copies of "Snow White", sold for 30 days, and there are 120 copies left. How many copies are sold per day on average?
新华书店运来540本《白雪公主》，卖了30天，还剩120本．平均每天卖多少本？
<gadget id="calculator">540 - 120</gadget>
<output>420</output>

<gadget id="calculator">420 / 30</gadget>
<output>14</output>

<result>14</result>