Life is complicated, which is why the best advice is often so incredibly simple you can remember it even when surrounded by the whirlwind that is everyday experience.

Take WayUp founder Liz Wessel's best tip for 20-somethings, for example -- it's simply "say yes." That might sound too minimalist to be helpful, but when you're faced with a tough choice of whether or not to accept a new opportunity and your head is spinning with pros and cons, complex decision-making frameworks are going to fall by the wayside. You will remember two little words, however, and they might actually help point you toward a specific course of action.

The most useful mental models and rules of thumb, in other words, are often dead simple, which is why an interview Adam Bryant of The New York Times did with Dropbox co-founder and CEO Drew Houston earlier this year is so helpful (hat tip to Business Insider for the pointer).

In the course of the fascinating discussion, which ranges from Houston's early computer obsession to his ideas about building a successful startup culture, Houston boils down his advice for ambitious young people into an incredibly simple if slightly quirky three-part formula: a tennis ball, a circle, and the number 30,000.

A tennis ball, a circle, and the number 30,000?

What could possibly be the meaning of this random-sounding list of things? As Houston explains to Bryant, these items represent his personal "cheat sheet" for those just starting their careers:

The tennis ball is about finding the thing you're obsessed with. The most successful people and successful entrepreneurs I know are all obsessed with solving a problem that really matters to them. I use the tennis ball for that idea because of my dog, who gets this crazy, obsessed look on her face when you throw the ball for her.

The circle is really about the idea that you're the average of your five closest friends, so make sure to put yourself in an environment that pulls the best out of you. And the last is the number 30,000. When I was 24, I came across this website that says most people live for about 30,000 days. So you have to make every day count.

Intrigued? Houston notes that he developed this "cheat sheet" when he delivered the 2013 commencement address at his alma mater, MIT. You can check out his complete remarks below for more details.