Here’s the poster from my local Sainsbury which displays what I shall call a “*calco*”, a mis-calculation, the numerical equivalent of “typo.”

Now £3.79. Was £4.29. Save 70p.

Actually that should be save 50p.

Or should it? All we can tell is that these numbers are not consistent with each other, and so any one could be wrong. Interestingly there’s no way of knowing on the face of it which of the numbers is wrong…might just need to buy some sausages and mash to find out.

But all is not lost. If we want to assume that only one of the three numbers is wrong, then by means of mathematical deduction, we can go as far to say that we would save 50p or 70p.

The fact that this error found its way on to the shop floor of a national chain is no mean feat. The poster will have been through some design, create, print, and probably approval process. Then distributed to stores and then shop floor and no one noticed this simplest of errors. Especially when you consider that the primary objective of the poster would appear to be to make a specifically numerical point.

Also makes me think that perhaps we need a “maths-checker”, the numerical equivalent to a spell checker. It would identify the numbers in prose and check for some sort of mathematical relationship.