After the 20 athletes have counted, the 10th to 20th will withdraw, and the rest will stay. How many athletes are left?
20名运动员报数后，10～20号退出，其余的留下，留下的运动员有多少人．
<gadget id="calculator">20 - 11</gadget>
<output>9</output>

<result>9</result>