Today’s math problem looks simple, but a lot of people get tripped up. It comes from Daniel Kahneman’s book Thinking Fast and Slow. Sharpen your pencils and let’s get to work.

A bat and ball together cost \$1.10. The bat costs one dollar more than the ball. How much does the ball cost?

## The horse problem

We like to shoehorn in a separate problem to make sure you don’t mistakenly scroll right down to the answer. Here’s a similar word problem, but instead of buying a bat and ball, you’re trading horses.

## Back to baseball

In the bat and ball problem, the most common incorrect answer is the ball costs 10 cents. The bat costs \$1.00, the ball costs 10 cents, that equals \$1.10. Right?

Not right!

In this problem, the bat has to cost \$1.00 more than the ball. If the ball costs 10 cents and the bat costs \$1.00 more, the bat costs \$1.10, so together they’d be \$1.20.

The correct answer is 5 cents. The ball costs 5 cents, the bat costs \$1.05, and together they cost \$1.10.

If you got it right, you’ve got something over half of students at some pretty fancy colleges. As Kahneman explained, “Many thousands of university students have answered the bat-and-ball puzzle, and the results are shocking. More than 50% of students at Harvard, MIT, and Princeton gave the intuitive — incorrect — answer.”