The rule of "2"

When I was a student on a limited budget, I had some idea, a hunch, that on average, I spent $2 on an item of food at the supermarket. By counting the number of items I purchased, including more than one of the same item, that I would have a fairly accurate idea of the grocery bill before I reached the checkout.

But to apply this rule, I found I had to apply it to every item, including each of the many 33-cent bags of Oriental noodles, each can of soup, each apple and orange, and so on. It saved me a lot of mental effort in having to compute the real total, and I found that I could always stay on budget with this rule.