There are 4 rows of desks in the classroom, with 12 desks in each row. 39 desks have been set up. How many desks are left?
教室有4排课桌，每排12张，已经摆好了39张，还剩下多少张没有摆好？
<gadget id="calculator">12 * 4</gadget>
<output>48</output>

<gadget id="calculator">48 - 39</gadget>
<output>9</output>

<result>9</result>