I've taken to tipping 18%, and the process I use to calculate that mentally starts with what you mentioned, but with one more step:
Divide by 10 for 10%; double that number for 20%. Take 10% away from that number to get 18%.
E.g.
100 check
move the decimal => 10
double that => 20
move the decimal => 2
subtract previous value from the one before (thus 90% of 20%, or 18%) => 18
This feels like a good mental math exercise to me on a 4- or 5-digit number since it requires a couple of values to be remembered while you're calculating. Gotta stay sharp!
Divide by 10 for 10%; double that number for 20%. Take 10% away from that number to get 18%.
E.g.
This feels like a good mental math exercise to me on a 4- or 5-digit number since it requires a couple of values to be remembered while you're calculating. Gotta stay sharp!