15 students are going to the other side of the river, there is only one small ferry boat, and only 3 students can take on the boat. Ask at least how many times can all be crossed?
15个同学要去河对岸，只有一只小渡船，船上只能乘3个同学．问最少用几次可以全部渡完？
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>