There are 47 tourists who want to cross the river. Now there is only one small boat, which can only carry 6 people at a time (without boatmen), and it takes 3 minutes to cross the river each time. So, at least how many minutes will it take to cross?
有47名游客要渡河．现在只有一条小船，每次只能载6人(无船工)，每渡河一次需要3分钟．那么，至少要花多少分钟才能渡完？
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">18 + 1</gadget>
<output>19</output>

<gadget id="calculator">19 * 3</gadget>
<output>57</output>

<result>57</result>