You’ve seen this riddle: a cow changing hands, sums of money adding up and subtracting… and in the end, your mind gets confused. You think you have the solution, then a detail sows doubt. What if, instead of trusting intuition, you applied a truly reliable method? Spoiler: no need to be a mathematician, just line up the steps like you line up your products in the supermarket…
Why this problem confuses us
The trap with this kind of puzzle is that it confuses two things: money going out (purchases) and money coming in (sales). Our brain mostly remembers the big numbers and ends up “rounding” or combining the data in its own way . As a result, we believe that the second operation cancels out the first, or that we have to add up all the amounts in a block. Wrong! Each purchase and resale is a mini-story with its own beginning and end.
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Quick reminder: profit, expense and cash flow
To stay calm , we use the basic trick: profit is what comes in less what goes out over a complete sequence. In other words, we never calculate a gain at the time of purchase (which is an expense), but at the time of sale (which confirms the value). Like at a garage sale: as long as the object is not resold, the gain does not really exist.
Step by step: unraveling the cow riddle
We follow the thread, calmly:
- You buy the cow for €800. At this point, there’s no profit, just an outflow of money.
- You sell it for €1,000. This first “buy + sell” sequence results in: €1,000 – €800 = +€200.
- You buy it back for €1,100. Again, this is just an expense; no gain at this point.
- You resell it for €1,300. Second complete sequence: €1,300 – €1,100 = +€200.
Add up the profits of the complete sequences (and only them): €200 + €200 = €400. Yes, that’s it! We don’t mix purchases between sequences, we don’t average prices , we respect the natural order: each sale validates the profitability of the previous purchase.
The classic trap to avoid
Many people think that buying back at €1,100 “eats up” the first €200 profit. In reality, it simply initiates a new transaction. Imagine two successive train tickets: one cost you €800 and brought in €1,000, the other cost you €1,100 and brought in €1,300. Each journey has its own profitability; you don’t mix the tickets to recalculate the overall route. Moral: we reason in complete blocks “buy → sell”.
Practical tip to avoid making mistakes
When a problem involves multiple round trips of money, draw two columns on a corner of a sheet of paper: Outflows (purchases) and Inflows (sales). Then group the transactions into logical pairs. Here:
Sequence 1: entry €1,000 – exit €800 = +€200.
Sequence 2: entry €1,300 – exit €1,100 = +€200.
Then, add up the profits from the sequences: +€400 in total. It’s as simple as whipping egg whites: step by step, without rushing.
In a clear and quantified summary
First transaction: €800 → €1,000 = +€200.
Second transaction: €1,100 → €1,300 = +€200.
Total profit: €400.
Keep this method handy: as soon as the numbers get tangled up, break the story down into small scenes… and the solution emerges naturally!
