There are 9 copper coins, one of which is fake. The quality of the real and the fake is only different. With a balance without weights, you can definitely find out the fake copper coin at least how many times you weigh it.
有9枚铜钱，其中一枚是假的，真假只是质量不同，用无砝码的天平，至少称多少次，就肯定能够将假铜钱找出来．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<result>3</result>