ah! don’t beat yourself up . I went to a grammar school , and was the smartarse who helped all the other kids with their maths. Like I said, misconceptions re. proprtion are extremely common. You only need what we call GCSE level maths for this, but only the top graders ever seem to actually get this kind of thing right.
You just have to bear in mind that adding, say 50% to a quantity is very different from subtracting same from the result.
That’s lso useful to bear in mind when going round the supermarket! " Two -for-the-price of one" is actually a much better deal than "50% extra free " though they look the same at first glance. With the “50% extra free”, you’re only getting one third of your purchase for free ! For a more direct comparison, “Two-for-the price of one” might legitimately be called “100% extra free” . But that could lead to a bit of confusion , couldn’t it? with the mathematically naive thinking that means they don’t have tp pay anything 