Mind you, shops delibrately confuse
In supermarkets, there is supposed to be a representative price on the tickets to allow easy comparison, like so many pence per amount of weight.
Unfortunately, my local T***o appears to deliberately compare different weights on the tickets, so one item will be priced per 250g, and another will be priced per 100g (and I have seen worse ones involving amounts like 330g, 350g).
There is only one purpose in this, and that is to defeat the measures introduced to allow product comparison.
I'm pretty good a mental arithmetic still, but sometimes I have to think for a few seconds before deciding which item is the best value. Other people wanting best value tend to just go for the supermarket brand, believing it will always be cheaper. More often than you may think, they are wrong.
I'm not saying that it's not a poor reflection on the education system nowadays, but it is clear that shops make it deliberately difficult.