Among the 17 silver dollars, one is fake, and its appearance is not different from the real silver dollar except that it is slightly lighter than the real silver dollar; weighing it with a balance without weights at least how many times is guaranteed to find the fake silver dollar.
在17个银元中，有一个是假的，除比真银元稍轻而外，其外表与真银元无任何差别；用一架无砝码的天平至少称多少次就可保证找出假银元．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<result>3</result>