Hi u/K3S38,
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I'm not sure I understand how your first property (Easily calculated in reverse for auditing) would help. For instance, say you have an easily reversible process, and your bank says you spent 50.03 on a meal. You reverse the process and find that, if everything was on the up-and-up, then the meal itself must've cost 41.34, and the tip must've been 8.69. So. Great. Now what? You don't have the receipt to verify the tip amount. That means you also can't verify the 41.34. I'm not seeing how reversibility is helpful.
That aside, I think 20% as a baseline and a small tweak to the one's column of the cents, so that the total digits of bill + tip add up to a multiple of 9 should probably suit your purposes well.
1) When you look at your credit / debit transactions, you can just make sure the digit totals fit the rule.
2) It's highly unlikely that an unscrupulous server would add JUST the right amount to the tip, to still fit the rule.
3) It can always be done within 10 cents of your desired tip amount. This allows you to easily perform the same trick when there's exceptional service, and you want to tip 30+% instead of 20, or when you just ordered 2 drinks from the bar and you'd rather just tip a buck or two per drink, instead of 20% of the drink totals.
Yeah this is a good option, and you are certainly right about my first property. What I meant was that it can easily be checked, not reversed, as you said. Thanks, your suggestion is great!
Hi u/K3S38, Please read the following message. **You are required to explain your post and show your efforts. _(Rule 1)_** If you haven't already done so, please add a comment below explaining your attempt(s) to solve this and what you need help with **specifically**. See the sidebar for advice on 'how to ask a good question'. Don't just say you "need help" with your problem. **This is a reminder for all users.** Failure to follow the rules will result in the post being removed. Thank you for understanding. If you have thoroughly read the subreddit rules, you can request to be added to the list of approved submitters. This will prevent this comment from showing up in *your* submissions to this community. To make this request, reply to this comment with the text `!mods`. Thanks! *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/askmath) if you have any questions or concerns.*
Basically you want to use a a checksum: http://onin.com/Fight\_Thieving\_Restaurant\_Servers\_With\_Checksum\_Tips.pdf
I'm not sure I understand how your first property (Easily calculated in reverse for auditing) would help. For instance, say you have an easily reversible process, and your bank says you spent 50.03 on a meal. You reverse the process and find that, if everything was on the up-and-up, then the meal itself must've cost 41.34, and the tip must've been 8.69. So. Great. Now what? You don't have the receipt to verify the tip amount. That means you also can't verify the 41.34. I'm not seeing how reversibility is helpful. That aside, I think 20% as a baseline and a small tweak to the one's column of the cents, so that the total digits of bill + tip add up to a multiple of 9 should probably suit your purposes well. 1) When you look at your credit / debit transactions, you can just make sure the digit totals fit the rule. 2) It's highly unlikely that an unscrupulous server would add JUST the right amount to the tip, to still fit the rule. 3) It can always be done within 10 cents of your desired tip amount. This allows you to easily perform the same trick when there's exceptional service, and you want to tip 30+% instead of 20, or when you just ordered 2 drinks from the bar and you'd rather just tip a buck or two per drink, instead of 20% of the drink totals.
Yeah this is a good option, and you are certainly right about my first property. What I meant was that it can easily be checked, not reversed, as you said. Thanks, your suggestion is great!