80% missed at least one of the questions

The following questions are known as the Cognitive Reflection Test by Shane Frederick (2005).

Can you answer them correctly?

(1) A bat and a ball cost $1.10 in total. The bat costs $1.00 more than the ball. How much does the ball cost? _____ cents

(2) If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets? _____ minutes

(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? _____ days

In a survey of 3,428 people, an astonishing 33 percent missed all three questions. Most people–83 percent–missed at least one of the questions.

Even very educated people made mistakes. Only 48 percent of MIT students sampled were able to answer all the questions correctly.

Give it a try before reading the answers.

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(1) 5 cents (not 10)
(2) 5 minutes (not 100)
(3) 47 days (not 24)

Extra explanation

(1) A bat and a ball cost $1.10 in total. The bat costs $1.00 more than the ball. How much does the ball cost? _____ 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 5 cents and the bat costs $1.05

(2) If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets? _____ minutes

If it takes 5 machines 5 minutes to make 5 widgets, then it takes 1 machine 5 minutes to make 1 widget (each machine is making a widget in 5 minutes). If we have 100 machines working together, then each can make a widget in 5 minutes. So there will be 100 widgets in 5 minutes.

(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? _____ 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.

 

 

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Bridge riddle

Locker riddle

The simple riddle that 50% of Harvard students get wrong..

for the other two question click here

Einstein’s riddle

The situation

  1. There are 5 houses in five different colors.
  2. In each house lives a person with a different nationality.
  3. These five owners drink a certain type of beverage, smoke a certain brand of cigar and keep a certain pet.
  4. No owners have the same pet, smoke the same brand of cigar or drink the same beverage.

The question is: Who owns the fish?

Hints

  • the Brit lives in the red house
  • the Swede keeps dogs as pets
  • the Dane drinks tea
  • the green house is on the left of the white house
  • the green house’s owner drinks coffee
  • the person who smokes Pall Mall rears birds
  • the owner of the yellow house smokes Dunhill
  • the man living in the center house drinks milk
  • the Norwegian lives in the first house
  • the man who smokes blends lives next to the one who keeps cats
  • the man who keeps horses lives next to the man who smokes Dunhill
  • the owner who smokes BlueMaster drinks beer
  • the German smokes Prince
  • the Norwegian lives next to the blue house
  • the man who smokes blend has a neighbor who drinks water

Einstein wrote this riddle this century. He said that 98% of the world could not solve it.

Solution to the Einstien’s RiddleSolution to the Einstien’s Riddle