## Archive for February 4, 2015

### 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.)

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

Which has to make you wonder:

• How picky are they, really?

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

## MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.