If Johnny has 6 apples in one and and 8 apples in the other, what does he have? The best answer is “very big hands” although 14 apples would also be correct. In day to day living, the ability to do simple mental arithmetic is invaluable as a way of making sure you get a good deal.
For example, I popped down to town this morning and Jane asked me to pick up a bottle of eyewash. I popped into Superdrug and the options included own-brand, at £2.99 and Optrex at £6.59, in similar sized boxes. Of course, eye wash comes in glass or plastic bottles and the boxes are just for display and protection of the contents. I looked more closely at the volume of eye wash for each: 110ml and 300ml.
With mental arithmetic, it is quite acceptable to round things to easy to handle numbers. That provides an estimate and you can always make an adjustment at the other end. So, own-brand at ~£3 per ~100ml gives ~£9 for ~300ml (or £8.97 for 330ml if you want to be precise). Suddenly it becomes obvious that this is significantly more expensive than the Optrex.
Having done my head-sums, there was only one sensible solution – buy two lots of Optrex (after checking the expiry date was a couple of years in the future).