World's shortest IQ test has just three questions – yet 80% of people fail it

Testing cognitive ability doesn't have to be long or complicated, as shown by a test published by professor Shane Frederick in 2005 that consists of just three questions. Despite its apparent simplicity, more than 80% of people who take it fail to pass.

When Frederick originally tested 3,000 participants, it turned out that only 17% of them were able to answer all three questions correctly. The test is considered one of the shortest ways to assess a person's ability to think logically rather than settle for the first seemingly obvious answer.

The three questions

The first question states that a bat and a ball cost $1.10 together, and the bat costs $1 more than the ball. How much does the ball cost? The creators of the test note that the first instinct – 10 cents – is usually wrong, and the question requires closer thought.

The second question asks: if five machines take five minutes to make five products, how long would it take 100 machines to make 100 products? Many find this task reminiscent of school-era maths lessons.

The third question involves water lilies in a lake that double in number every day. If the lilies cover the entire lake after 48 days, how many days would it take for them to cover half the lake?

Since its publication, the test has become popular as a quick way to check whether people can resist their first intuitive but often mistaken answer and apply logical reasoning instead.