There are 16 people who want to take a boat to the other side of the river. There is a self-rowing boat at the ferry, and this boat can only take 4 people at a time. To send all 16 people to the other side, how many times does this boat have to go back and forth at least? (A back and forth counts as 2 times).
有16人要坐船到河对岸去，渡口有一条自划船，这条船每次只能坐4人．要把16人都送到对岸去，这条船至少要来回渡几次？(一个来回算2次)．
<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 1</gadget>
<output>9</output>

<result>9</result>