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?

I’ll give you a moment to work on your answer.

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.

You can find the answer to the horse problem here.

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.”

See Also: Can you solve this math problem that went viral?