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。

推荐阅读：
Back to the Basics: How to Get More Twitter Followers　 Trump Steals Spotlight During Democratic Debate　 Dorsey’s 8% Twitter Layoff　 Finding Out Which Content Is Unacceptable For Your Website　 The Custom Sort String Algorithm with Count and Write　 Algorithms to Count the Number of Palindromic Substrings　 Depth First Search Algorithm to Find Leaves of a Binary Tree　 Understanding The Math Before Investing In Stocks　 Don’t Owe A Thing: Tips for Bloggers Filing Taxes　 The Quirky Quad Targets Millennials