From July 15th to August 4th, Zhang Jie wrote an average of 24 big characters every day. How many big characters did he write in total?
张洁从7月15日到8月4日，平均每天写24个大字，他一共写了多少个大字？
<gadget id="calculator">31 - 15</gadget>
<output>16</output>

<gadget id="calculator">16 + 1 + 4</gadget>
<output>21</output>

<gadget id="calculator">24 * 21</gadget>
<output>504</output>

<result>504</result>