## Saturday, September 25, 2010

### Tipping Checksum

Recently, I have become aware that some restaurant employees pad their tips by adjusting upward the value on the tip line and total (even at some nice places!). For the most part, the increases are for \$1 or \$2. I don't go back and match every single credit card receipt to the credit card statement line items, so it would be nearly impossible for me to notice this.

Starting today, I am going to try something new. And, depending on whether or not it inconveniences me or not, I may make it a permanent practice.

I will tip in such a way that the sum of the digits in the final total will be 9 (mod 10) -- basically the last digit of the sum will be a 9.

Quick examples...

Example 1

Pre-Tip Total = \$49.74
Round Tipping (my current practice) = \$8.00
Total = \$57.74, which has a sum of 5+7+7+4 = 23. [3 (mod 10)]

So, I will want to subtract 4 cents.

New Tipping = \$7.96
New Total = \$57.70, which has my desired sum that is 9 (mod 10)

Example 2

Pre-Tip Total = \$79.16
Round Tipping (my current practice) = \$12.00
Total = \$91.16, which has a sum of 9+1+1+6 = 17. [7 (mod 10)]

So, I will want to add 2 cents.

New Tipping = \$12.02
New Total = \$91.18, which has my desired sum that is 9 (mod 10)

Of course, roughly 10% of the time I will not need to adjust my round tip. And, that's completely fine, as any future adjustment to the value will likely break my checksum constraint.

Once I begin this practice, it should be relatively easy to look at credit card line items for restaurant charges (excluding fast food). Also, since I normally tip on take-out orders, I won't need to differentiate between dine-in and take-out line items.

In any case, I guess I'll have to wait and see if this is going to be annoying to keep up or not.