Boys and girls line up hand in hand, and there is a girl between every two boys. If there are 10 boys, how many girls are there?
男生和女生手拉手排成一行，每两个男生之间都有一个女生．如果男生有10人，女生有多少人．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>