Apparently 80 per cent of people fail this "easy" three question IQ test

Publish Date
Wednesday, 24 July 2019, 11:07PM

The world's shortest IQ test has just three questions, yet fewer than one in five people can answer all three correctly.

Dubbed the Cognitive Reflection Test, it comes from a 2005 paper by MIT professor Shane Frederick, who sought to demonstrate the difference between fast thought processes that occur with little conscious deliberation, and those that are slower and more reflective.

The test consists of three brain teaser-type questions that are harder than they first appear, where the immediately obvious "right" answer is actually incorrect — if you stop and think about it, reports.

"The three items on the CRT are 'easy' in the sense that their solution is easily understood when explained, yet reaching the correct answer often requires the suppression of an erroneous answer that springs 'impulsively' to mind," Prof Frederick wrote.

Out of 3,428 people who took the test, just 17 per cent answered all three questions correctly, more than half got at least one wrong, and one third scored zero out of three. 

Here are the questions:

  1. A bat and a ball cost $1.10 in total. The bat costs $1 more than the ball. How much does the ball cost?
  2. If it takes five machines five minutes to make five widgets, how long would it take 100 machines to make 100 widgets?
  3. In a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?

Below are the answers most people guess, and then the correct answers ...

• This brainteaser is baffling the internet! How long does it take you to spot the cocktail umbrella?

What most people guess:

  1. 10 cents
  2. 100 minutes
  3. 24 days

As Prof Frederick noted, the intuitive answer to the first question is 10 cents, but this "impulsive" answer is wrong.

"Anyone who reflects upon it for even a moment would recognise that the difference between $1 and 10 cents is only 90 cents, not $1 as the problem stipulates," he wrote.

"In this case, catching that error is tantamount to solving the problem, since nearly everyone who does not respond '10 cents' does, in fact, give the correct response."

Here are the correct answers:

  1. 5 cents:
    Say the ball costs X. Then the bat costs $1 more, so it is X + 1. So we have bat + ball = X + (X + 1) = 1.1 because together they cost $1.10. This means 2X + 1 = 1.1, then 2X = 0.1, so X = 0.05. This means the ball costs five cents and the bat costs $1.05.
  2. 5 minutes:
    If it takes five machines five minutes to make five widgets, then it takes one machine five minutes to make one widget (each machine is making a widget in five minutes). If we have 100 machines working together, then each can make a widget in five minutes. So there will be 100 widgets in five minutes.
  3. 47 days:
    Every day FORWARD the patch doubles in size. So every day BACKWARDS means the patch halves in size. So on day 47 the lake is half full.

Interestingly, men scored "significantly higher" than women on the CRT.

How many questions did you get right?


This article was first published on the NZ Herald and is republished here with permission.