Money-Saving Fermi Questions

I was pissed when my cousin wouldn’t give me two $5 bills for a $10 bill.

“Sorry, can’t,” he replied simply.

When asked why the hell not — I knew he had two $5 bills, because he had gotten one from the gas station attendant earlier, and the waitress just brought him another — he explained that all $5 bills are put into savings.

“When I receive a $5 bill, I don’t spend it. It stays in my wallet till I get home, and then it goes right into the piggy bank,” he said. “Every couple months, I take those bills to the bank. It’s an easy way to build up my savings account.”

“So, what, you save like $50 a year this way?”

“It’s a helluva lot more than you’d think,” he replied.

As stupid as this sounds, now everyone in my family is doing it. It is a low-impact way to build up your savings account. And it leads to a great Fermi question:

• If all of your $5 bills go into savings, how much will you save in a year?

And for my sons, who don’t often pay for things with bills large enough to require $5 in change, we have the following:

• If all of your nickels go into savings, how much will you save in a year?

Fermi questions are questions that require quantitative estimates to arrive at an answer. It often requires making assumptions, because exact data is unavailable. Here are a few others:

• What percent of people who have ever lived are currently alive?
• How many hot dogs are sold at Yankee Stadium during a baseball season?
• How long would it take a snail to travel from Miami to Los Angeles?
• What is the weight of a million dollars? (Assume 1,000,000 one-dollar bills.)

My favorite Fermi question is based on a Dunkin Donuts radio advertisement, in which they boasted:

We reject more than one million pounds of coffee beans a year.

Which has to make you wonder:

• How picky are they, really?