21 people go to the other side of the river. There is only one small boat by the river, and each time the boat can only accommodate 4 people. How many times does the boat carry at least before all of them can cross the river.
21人到河的对岸去，河边只有一条小船，每次船上只能坐4人，小船至少载多少次才能全部过河．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<result>7</result>