45 people want to cross the river. There is only one boat at the ferry that can carry no more than 5 people. They all cross the river. How many times do they need to use this boat at least? (A trip back and forth is twice).
45人要过河，渡口只有一只没有船工且最多只能载5人的小船，他们全部渡过河，至少要使用这只小船多少次？(来回一趟是两次)．
<gadget id="calculator">45 - 5</gadget>
<output>40</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<result>21</result>