World’s shortest IQ test stumps almost everyone with just three questions

What does it mean to be truly intelligent? While IQ tests have long been used to measure cognitive ability, intelligence is often more about problem-solving and reasoning than test scores. A simple yet surprisingly effective way to assess logical thinking is the Cognitive Reflection Test (CRT)—a three-question challenge that has puzzled test-takers for years.

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Yale professor Shane Frederick introduced the CRT in a 2005 study published in the Journal of Economic Perspectives. Unlike traditional IQ tests, this one aims to determine whether a person relies on instinctive responses or engages in deeper analytical thinking. The questions appear straightforward, but their deceptive nature often leads to incorrect answers. Here they are:

1. A bat and a ball together cost $1.10. The bat is $1.00 more expensive than the ball. How much does the ball cost?
2. If five machines take five minutes to produce five widgets, how long would 100 machines take to make 100 widgets?
3. A lake contains a patch of lily pads that doubles in size every day. If the lake is fully covered on day 48, when was it half-covered?

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The test’s simplicity is what makes it so tricky. According to Frederick’s research, the correct answers are 5 cents, 5 minutes, and 47 days. However, many people instinctively arrive at the wrong conclusions, demonstrating the difference between quick intuition and deliberate reasoning.

A detailed breakdown of the answers shows why so many get them wrong. For the first question, a common mistake is assuming the ball costs 10 cents. That response seems logical at first glance but doesn't hold up mathematically. If the ball were 10 cents, the bat would be $1.10, making the total $1.20 instead of $1.10. Setting up an equation—x + (x + 1) = 1.10—solves for x = 0.05, meaning the ball costs 5 cents.

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The second question follows a similar pattern of misinterpretation. Some test-takers assume that if 5 machines take 5 minutes, then 100 machines must take 100 minutes. In reality, productivity scales proportionally. If 5 machines make 5 widgets in 5 minutes, then 100 machines—working at the same rate—would still complete 100 widgets in 5 minutes.

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Perhaps the trickiest question of all is the lily pad problem. A frequent mistake is to assume the lake was half-covered at the halfway point—24 days. However, since the patch doubles each day, the correct answer is 47 days, because on the final day, the patch simply doubled from its previous size.

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The original study conducted by Frederick gathered responses from 3,428 participants across 35 studies over a span of 26 months. Many of the test-takers were university students from various institutions, receiving $8 to complete a 45-minute questionnaire. The test was designed to measure time preference and risk preference, revealing how people make decisions based on instinct versus analysis.

The test aligns with the research of Nobel Prize-winning psychologist Daniel Kahneman, who identified two modes of thinking. System 1 is fast and automatic, leading to quick but often incorrect answers. System 2 is slower, requiring effort and analysis, but producing more accurate results. Participants who immediately responded with 10 cents, 100 minutes, or 24 days were engaging in System 1 thinking, while those who paused to think through the logic correctly used System 2.

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Beyond just answering test questions, cognitive ability is linked to real-world outcomes. According to Frederick, individuals with strong cognitive skills tend to earn more, have better working memory, live longer, and react faster. While traditional IQ tests remain a benchmark for intelligence, the CRT offers a fascinating glimpse into how people process information and make decisions.