About a week ago I became a proud owner of a Roomba robot vacuum cleaner (the 880 model, to be exact). I’m not rich by most standards, but when my wife suggested we get this, I wrote down a few numbers on a napkin, and immediately said yes to dropping $500 on a robot vacuum cleaner.

Why? Because I’ll actually earn money from buying this expensive robot cleaner because of a simple business principle and some math that I picked up from the 4-Hour Work Week by Tim Ferriss. Here’s how it works:

I spend roughly one hour each week vacuuming (usually in two half-hour segments).

52 weeks a year X 1 hour per week = 52 hours a year

I’m thinking if I’m lucky, the Roomba will last me at least 5-7 years. That means the Roomba will save me:

52 hours a year X 5 years = 260 hours

At $500, that means that the Roomba costs me just $1.92 every hour. Or another more futuristic way of thinking about it could be I’m paying the Roomba a little under two bucks per hour to clean my house. As a musician it’s very hard for me to figure out my hourly pay, but I’m pretty sure I make way more than $1.92 per hour.

There’s a vital business principle behind this purchase: anytime you can pay someone less per hour than you do to handle a task for you, do it.

Your time is incredibly valuable. Focus on using it to do things only you can do, and leave the vacuuming to the robots.