题面摘自 Henry E. Dudeney 公版文本；英文为古腾堡原文整理，中文为本站自译，提示、解答骨架和闲谈保留本站原创结构。
Here is a little tangle that is perpetually cropping up in various guises. A cyclist bought a bicycle for £15 and gave in payment a cheque for £25. The seller went to a neighbouring shopkeeper and got him to change the cheque for him, and the cyclist, having received his £10 change, mounted the machine and disappeared. The cheque proved to be valueless, and the salesman was requested by his neighbour to refund the amount he had received. To do this, he was compelled to borrow the £25 from a friend, as the cyclist forgot to leave his address, and could not be found. Now, as the bicycle cost the salesman £11, how much money did he lose altogether?
这是一个以各种形式不断出现的小混乱。一名骑自行车的人花 15 英镑买了一辆自行车，并支付了一张 25 英镑的支票。卖家找到附近的店主，让他帮自己兑换支票，骑自行车的人收到了 10 英镑的零钱后，登上机器就消失了。事实证明，这张支票毫无价值，推销员的邻居要求他退还收到的金额。为了做到这一点，他不得不向朋友借了 25 英镑，因为骑自行车的人忘记留下他的地址，而且无法找到。现在，由于这辆自行车花费了推销员 11 英镑，他总共损失了多少钱？
提示 1
先说出现象：哪些量会变，哪些约束不会变。
提示 2
找守恒量、相似关系、平衡条件或不变量，不急着代公式。
提示 3
把物理图景或谜题结构翻成一个最小方程组，再处理边界情况。
完整解答
解题主线是先把 Dudeney 谜题 8 的条件整理成一个稳定模型，再选择最少的变量。第一步确认约束，第二步写出关键关系，第三步检查特殊情形。这里给的是原创解法骨架；若要核对原始题面，请回到公版来源。
这类题最怕一上来套公式。先把图景或语言条件说清楚，答案通常会少绕很多路。