First, an introduction. When you’re at a restaurant, and you pay by credit card, how do you decide what to tip? That is, how do you choose an exact amount that’s within the acceptable range? Two common strategies are 1) to choose a round number for a tip and add it to the (not round) total, or 2) choose a value for the tip that will make the final amount a round number.

I have heard that one way is less secure, though I can’t remember which, because the person running the bill can input another amount and you wouldn’t know just from checking your credit card statement. I don’t know how credible this threat is. But now I happen to have a solution.

You can use a checksum for your checks. Leave a final amount such that the last digit is equal to the sum of all the digits preceding it. For example, instead of paying $42, leave a tip such that the amount you pay is $42.06. This method is straightforward, awesome, and helps curtail credit card fraud.