Many people don't understand percentages.
As I've written to Harry, I think that most people 'understand' percentages, in the sense that the find them more easily conceptualised and thought about than fractions. I suppose that makes sense, in as much as one is essentially converting fractions to ones that all had a common denominator, enabling one to compare chalk with chalk, rather than with cheese.
For example, I suspect that many, even very 'numerate', people could not tell you, at a glance (i.e. with at least doing some mental arithmetic), whether, say, 7/17 was more or less than 5/16. However, if one converted that to (approximately, rounded), 41.2% and 31.3% the answer would be obvious to almost anyone. Even more dramatic if the fractions were, say, 59/171 and 76/211
What a lot of people understand far less is how to 'apply' percentages - in particular, as you go on to illustrate, getting the numerator and denominator 'the right way around'.
I remember once having quite a job to explain to someone, in the context of moaning about an example of shrinkflation (a despicable practice, IMO) where they'd reduced the size of a product from 600ml to 500ml and kept the price the same was a 20% price hike, not the 16.7% he'd come up with., respectively,
That's actually quite a subtle one, and a very common sort of error. In fact, I suspect that an appreciable proportion of even fairly 'numerate' people would come up with the incorrect answer 'intuitively', even if they would produce the correct answer if they 'stopped and thought about it'.
Consumers can sometimes be exploited when two or more 'percentage discounts' are applicable to a purchase, since the order in which the discounts are applied will affect the total amount of discount they agree - so guess what order the sellers usually use

.
...And how many times have you seen a doubling of something as a "200% increase"?
Very often, to the extent that if they say "200% increase", one really can't be even remotely sure as to whether they actually mean a 'doubling' opr 'trebling'. However ......
Or where an increase in a probability from 10% to 15% is described as a 5% increase?
That's a more complicated one, and one which I personally often have to deal with quite often, and where standard terminology/language does not necessarily result in clarity!
There are plenty of quantities which are normally expressed in 'units' of "%" - things like relative humidity, inflation, interest rates etc. etc. and, in the medical field, things like HbA1c, many respiratory function tests, some 'blood counts', % surviving at X months etc. etc.
If a quantity increases from 200 Volts to 220 Volts, we would describe that as an increase of 20 Volts - and similarly if the increase was from 200 metres to 220 metres, 200 degrees to 220 degrees or whatever. However, if the 'unit' of the quantity is "%", then a statement such as "an increase of 10%", this is quite likely to be taken to mean (as per your thinking above) as meaning a
10% increase in the percentage. In many cases, that is not the 'change'/'difference' of interest, actual interest being in the 'absolute change in percentage'. When I'm dealing with this, I have little choice but to explicitly talk about "absolute change/difference in percentage", but that's rather long-winded and 'messy'!
Another problem with 'percentage changes/differences' is that they are not infrequently talked about without the necessary consideration of the absolute magnitudes of the changes/differences concerned. We've seen that with various "Pill scares" and "Vaccine scares". The revelation that something results in, say, a 50% (or even 100%) increase in the risk of death, or something else nasty, sounds pretty dramatic, and worrying. However, if (as has sometimes been the case) the "50% increase" has been an increase from "1 in 5 million" to "1.5 in 5 million", it is very questionable as to whether anyone should lose much sleep over it
