A racket and a ball cost 11 euros and the racket costs 10 euros more than the ball. How much does the ball cost? If you’re one us it’s very likely you have jumped straight to the conclusion that the ball costs 1 euro. Alas, that is the wrong answer. In fact, the ball costs only 0.5 euros, and that is the only reasonable answer considering that if the racket then costs 10.5 euros than it’s 10 euros more expensive than the ball and the two prices add up to 11 euros.
Many of us just can’t resist using the intuitive answer provided by the problem itself – i.e. the difference between 11 and 10 – and stumble upon a well known and researched behavioral bias (the case has been adapted by me from “Thinking, Fast and Slow” by D. Kahneman). The ones who gave the correct answer for some reason abstained from using the shortcut implied by the numbers provided in the case and took their time to work it out analytically.
If it’s really a live-or-die dilemma the one you’re facing, no matter how easy it seems to you take your time to figure out the correct answer and don’t use your false-guided intuition. After all it’s not a speed test on your intelligence, though it questions your cognitive biases.