Go confidently in the direction of your... math?

Since my math is not all that great, I use a little trick when I'm at the circulation desk and need to take a payment for a fine or lost book and am without a calculator. I fake it.

I don't mean that I shortchange people, and I don't mean that I miscalculate the fine. I just take a few seconds longer to do the math in my head and then write down the amount I've taken from the patron, and double check myself as I hand the patron their change.

This is something I've honed after years of working in libraries and in retail at a bookstore. If you act confident about the amount you're giving back, the patron usually won't check your math, and if they do and it's wrong, you can feign ignorance (or blame it on being an English major, which is what I usually do) but this type of error is rare, very rare, with my method. I should patent it.