I mean "when presented with a bill, how much to tip." It's just not the sort of thing which mathematicians are ever expected to do. There is no math journal whe

I mean "when presented with a bill, how much to tip." It's just not the sort of thing which mathematicians are ever expected to do. There is no math journal where you can publish the tips you calculated. It's not their job, there are no theorems to be proven there, etc., etc. (You're actually doing it a little more complicated than it has to be because it sounds like you're spending a little memory remembering the number you took 10% of. Take half, add it to the original, which is presumably still right in front of you, then drop the digit after.) I learned a lot of mental calculation because my background is in applied physics, where you're expected to reason about the size of things etc. (A good example question might be, "how far do you think an air molecule can go before it bumps into another air molecule?") So, just for example, to convert Celsius to Fahrenheit you multiply by 2, subtract 10%, and add 32. It is exact. So 25°C → 50 → 45 → 77°F. To convert Fahrenheit to Celsius you subtract 32, divide by 2, add 10%, then add 1% if you really feel like it; it is approximate. So 52°F → 20 → 10 → 11 → 11.11°C. Those sorts of things I will do in my head when people give me a number in one or the other temperature. Tipping is especially nice when compared to the fact that in New York State there is an 8% sales tax which is not added to any of the prices on stuff at the store -- calculate a tip and halve it. [Naive reasonings on air: small upper bounds can be given by the fact that it has to travel easily through the small passageways in your inner ear, but my favorite calculation is to take (kinematic viscosity) / (speed of sound), both of which are things that an applied physics geek should roughly know.]