11 Best Brain Teasers in the World

  1. Terry is half as old as Alice was when Alice was five years older than Terry is now. How old is Terry now?
  2. In 40 seconds or less without using calculator figure out which is greater, 354×357 or 355×356.
  3. “Many more people who smoke develop lung cancer than those who do not smoke” What research would possibly show that cigarette smoking does not cause cancer?
  4. What day follows the day before yesterday if two days from now will be Sunday?
    (a)notes: composer
    (b)sound: musical instrument
    (c) crayon: drawing
    (d)furniture: carpentry tools
    (e) symphony: piano
  6. A doctor’s son’s father was not a doctor. How is this possible?
  7.  What’s wrong with this advertisement: “Shop early and avoid the crowds.”
  8.  How could you figure out the meaning of the word INEXTRICABLE?
  9. Suppose a four-digit number is an exact multiple of 9 and three of the four digits are 1, 2, and 3. What is the fourth digit??
  10.  Which is greater, one half the surface area of a ball with radius 3” or the area of a circle with radius 3”?
  11.  If there are 24 people at a party and each person shakes another person’s hand, how many handshakes are there?


  1. Terry is 5 years old. Trick translate words to math a=5+t, a is alice’s age; t=(1/2)a where t is terry’s age  T=(1/2)(5+T)  =>2T = 5+T=>T=5
  2.  356×355 is greater. Calculate the last two digits by multiplying the digits in the unit’s places.
  3.  Look for something that does not link smoking to cancer directly, but indirectly. That is, something that causes one to smoke and the same thing that causes cancer. So research could find that there is a certain condition that causes one to smoke and the same conditions causes one to have cancer; it is the condition that causes it. Unfortunately, that is not the case.
  5.  (d)
  6.  Say IN means NOT, then associate EXTIC with another word, EXTRACT. So Inextricable means not extractable.
  7.  The doctor is the son’s mother.
  8.  If many people paid attention to the ad it would be crowded in early hours. Of course the reason why ad was put up was that the store did not think that that so many people would read the ad.
  9.  If a number is also a multiple of 9. So 1+2+3+x=9 and x=3.
  10.  One half the surface of a ball is greater.
  11.  276 Each person can shake hands with 23 people, so we have 24×23 but since it takes two people to make a handshake we must divide the result by 2.

PhD researcher at Friedrich-Schiller University Jena, Germany. I'm a physicist specializing in theoretical, computational and experimental condensed matter physics. I like to develop Physics related apps and softwares from time to time. Can code in most of the popular languages. Like to share my knowledge in Physics and applications using this Blog and a YouTube channel.
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