December 29, 2010 spout

# Enabling the Tip Calculator in Your Brain

I can’t imagine anyone reading this blog needs to read this, but I can’t help myself.

When I was just a wee lad, probably the most valuable thing I learned was how to perform mathematical estimation, the importance of which and several techniques you can get by reading Jon Bentley’s The Back of the Envelope (this essay along with several others, are collected in his most excellent books Programming Pearls and More Programming Pearls, both of which are still relevant a decade later). Not only is estimation generally quicker than running a calculator, but even when you do run a calculator, it helps you figure out when you did it wrong, the latter of which has saved my bacon time and again.

For example, as much as I love the Windows Phone 7 marketplace and it’s quality and quantity of applications, the ones that puzzle me are the tip calculator” apps (several!). I don’t understand why it’s worth the trouble of pulling out your phone and punching buttons when you can know the tip instantly.

For example, let’s assume the dinner bill is \$37.42. If the service was bad, that’s a 10% tip (you have to tip them something cuz the IRS assumes you will and taxes them accordingly — bastards). So, with a 10% tip, take the bill and move it right one decimal point: \$3.74. Now, round up or down depending on how bad the service was, e.g. \$3.50 or \$4. Quick and easy.

Assuming the service was great, that’s a 20% tip, so double the bill and move it right one decimal point, making the math easier for yourself, e.g. \$37.42 is close to \$35, doubling is \$70, so a \$7 tip. Boom: 20% tip.

If you want to get fancy and provide a 15% tip for good but not great, then average the two numbers: (\$4 + \$7)/2 = \$5.50. Zim zam zoom.

Honestly, as great as the apps are on your phone, tablet or BlueTooth headset (seriously), think about using the apps in your head first. Now only are they quicker and cheaper, but using them staves off dementia (which is a good thing!).

Oh, and if the tip is added as a mandatory minimum, then the additional tip is easy: \$0.00. I don’t deal well with authority.