Oh, that means it's correct! YAY! 😄 😄 😄
Umm, justifying, let's see...
1. water: y litres
milk: x litres
2. water: y - a
milk: x + a
3. water: y - a + b
milk: x + a - b
a: spoon full of water quantity
b: spoon full of water and milk quantity
3. w: y - a + b = y - (a - b) = y - (water - water + milk) = y - milk = water
m: x + a - b = x + water - water + milk = x + milk = milk
Don't know if that makes sense but.... 😕
Well... yeah, that DOES work, Evy, and you are right. The trick is that no matter what amounts or what stirring you do you bring back to the original glass an amount directly proportional to the amount you took out in a round trip so that the mixing is identical. So if you have two glasses of 20 units each, take 5 of one into the other, then you will see no matter what mix of 5 you take back the final mixing of the two glasses would be identical.
Anyone want another?
Ahhh... good, that's right, well done. Fell for one of two ways to read that wrong at first, I am afraid. The other mistake is to think that you make ten pounds when you sell it, lose ten when you buy it back and make another ten when you finally sell it for a ten pound profit, but both that and your original thinking involve mis-use of numbers.
Easiest and straightest way to avoid confusion is simply to total expenditure and income- you spend £160 and get £180 back, and suddenly there is no way for the puzzle to trick you.