15 children are lined up, there are 9 people in front of Xiaodong, how many people are behind Xiaodong?
15个小朋友排成一队，小东的前面有9人，小东后面有几人？
<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<result>5</result>