Three men go for a meal in a restaurant. At the end of the meal the waiter brings them the bill and it comes to £30. Each man puts in a ten pound note. The waiter rings it up and then realizes he made a mistake and the bill was actually for £25.
The waiter brings back five pound coins. As five doesn’t divide into three very well, the men take one pound each and leave two pounds as a tip which the waiter pockets.
Each man has now paid 9 pounds, which adds up to £27. The waiter has 2 pounds in his pocket. That adds up to £29.
What happened to the extra pound from the original £30?
Try as hard as you like to make the formula for that work and there will always be one pound missing.
The answer is that there never was an extra pound.
The three men paid £27 and the waiter took two leaving £25.
The way the conundrum is worded leads you to believe that there was £27 with £2 added to give £29, and therefore one pound short of £30, with £1 missing.
It is a good example of how easy it is to mislead somebody’s thinking so they mathematically “prove" something to be other than it is.
