Four Give Us Our Daily Bread

Jack goes into a supermarket and buys 4 rolls. The total price is £1.20. How much does eack roll cost?
Answer: 30p.
A baker's dozen would cost £3.90.

Jill goes into a supermarket and buys 5 rolls. The total price is £1. How much does each roll cost?
Answer: 20p.
A baker's dozen would cost £2.60.

Wrong.

It would cost £2.90 (i.e. £2 for 10 rolls at 20p each and 3 rolls at 30p each.).

Jill would clearly be better off buying 15 rolls for £3, and selling her two unwanted rolls to Jack at a discounted price.

This was the conundrum I was faced with today when I chose 4 rolls instead of 5. And being faced with a conundrum in the supermarket early on a Saturday morning is not the best start to the day. To circumvent the need for me to pay £1.20 for 4, an assistant was dispatched to fetch me an additional roll so I ended up with 5 for £1. The cashier could not challenge the computer system set up to manage this craziness by quietly agreeing I could leave the store with 4 rolls for a £1, I had to have the additional roll.

And the moral of this tale?

Firstly, Jack and Jill should have contented themselves with fetching a pail of water.

Secondly, more and more our behaviour is being shaped to fit the computer models rather than the models being adapted to suit the vagaries of human behaviour. But then, unlike the rolls, I'm a crusty old fool.