A group of young pioneers cross the river by boat. If each boat takes 15 people, there are still 9 people left. If each boat takes 18 people, there is just one boat left. How many boats are there?
一队少先队员乘船过河，如果每船坐15人，还剩9人，如果每船坐18人，刚剩余1只船，求有多少只船？
<gadget id="calculator">9 + 18</gadget>
<output>27</output>

<gadget id="calculator">18 - 15</gadget>
<output>3</output>

<gadget id="calculator">27 / 3</gadget>
<output>9</output>

<result>9</result>