Ex-Amazon employee and start-up CEO shares a surprising trick for getting ahead at work

When Manuel "Manny" Medina was a senior product manager at Amazon (AMZN) in 2003, he noticed something: Nearly all of the executives could solve difficult mathematical equations without using a calculator or even a pencil.

"The executives at Amazon and all the [vice presidents] were able to do math in their head really quickly," he says. "It was so stark, their ability to manage numbers so quickly."

Medina realized that if we wanted to get ahead, he needed to get more comfortable working with numbers in his head. He bought a book on speed math to "just keep up with them."

While having strong math skills is often a prerequisite for professionals in the tech or finance world, being comfortable with numbers can help employees in other industries, Medina says.

"Being fast with numbers propels you to a top position of authority," Medina tells CNBC, "because authority means authority over the facts."

After he worked for Amazon, Medina became an executive for Microsoft (MSFT). Today, he is the CEO of his own start-up, Outreach, an online platform for sales teams.

Knowing how to calculate percentages or subtract large numbers without a calculator helps you make better arguments and presentations, he says. For instance, if you wanted to quickly figure out what 5 percent of 235 is, you could move the decimal over one place to the left (23.5) and divide by two (11.75), which gives you your answer.

Additionally, being fluent in numbers and having the ability to recite industry statistics also can show you are an expert.

"It adds a level of credibility," he says. "At Amazon, this was very much baked into the culture."

Check out what Medina learned while working for Microsoft .

More From CNBC

• Top News and Analysis
  

• Latest News Video
  

• Personal Finance