That's basic numeracy, the kind people need every day in their lives. This widget costs $7.99, and I need 7 of them, how much is this going to cost me roughly

That's basic numeracy, the kind people need every day in their lives. This widget costs $7.99, and I need 7 of them, how much is this going to cost me roughly when I get up to the checkout counter? Is there enough cash in my wallet? That's ~$56, and I've got $60, will that cover the 5% additional sales tax? 10% of $56 is $5.60 (by the shift the decimal point method), half of that is $2.80. Guess I'm good. Being able to estimate and do mental math quickly is important for everybody, and the key to doing that is rules-of-thumb and having the basic facts memorized. Think of it as caching/memoization and heuristics, if that puts it in a more computer science frame.