21 expedition members cross a small river. There is only one boat that can carry 5 people. It takes 3 minutes to cross the river once. How many minutes does it take for all the team members to cross the river at least?
21名探险队员过一条小河，只有一条可乘5人的小船，过一次河需3分钟，全体队员渡到河对岸至少需要多少分钟．
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

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

<result>27</result>