群里“怪盗”同学提问：玩具店的玩具每当卖出一半时，就补充20个，到第十次卖出一半后恰好余下20个，则玩具店有玩具几个？

数学题解答：第十次卖出一半后恰好余下20个，这时尚未补充20个。如果是补充的20个，那末只剩下0个，这是不可能的。所以卖出一半，恰好余下20个，说明余下的20个是第九次补充之后的一半，也就是说第九次卖掉一半，并补充20个后是40个。所以每次卖掉一半，并补充20个，那么正好就和卖之前一样多。由此可以推算出：原来共有40个玩具。

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