Living on a shoe-string I stupidly mentioned to my mother that I would need to wait to the following month to buy a new pair of boot-laces.
She asked why, and I said I had very little housekeeping money left for the last fortnight of the month, so I had knotted my broken lace.
"That's ridiculous, " said she, "You simply divide a month's housekeeping money by 30, so you know exactly how much you can use every day."
I tried to explain that if I did that, I would never be able to buy more than a loaf of bread and a pint of milk a day, but she wasn't having that, even when I explained the figures behind the statement.