This apparently baffling logic puzzle is driving the internet crazy.
Designed for 14-year-olds pupils in the Singapore and Asian Schools Math Olympiads, it quickly went viral as millions of people struggled to even start working it out.
Then there were complaints over how it was worded, with many moaning that the grammar of the question was incorrect.
So can you solve the problem or are you also stumped by a question set for schoolkids?
Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is.
Cheryl gives them a list of 10 possible dates. May 15, May 16, May 19, June 17, June 18, July 14, July 16, August 14, August 15, August 17. Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.
Albert says: “I don’t know when Cheryl’s birthday is, but I know that Bernard does not know too.”
Bernard says: “At first I don’t know when Cheryl’s birthday is, but I know now.”
Albert says: “Then I also know when Cheryl’s birthday is.”
So when is Cheryl’s birthday?
SCROLL BELOW THE PIC FOR THE ANSWER….
It can’t be May 19 or June 18 because if Bernard had been told either of those days, he’d know the birthday immediately. However the only way Albert, who already knows the month, can be sure Bernard does not know the answer is if the birthday is in neither May or June. Once Bernard then tells us he didn’t know the birthday, but does now, we can discount both July 14 and August 14, because if he’d have been told it was on a 14th he’d still be none the wiser. In the same way, once Albert knows Bernard has worked it out, he knows the answer too. If he’d been told it was in August, he’d still be in the dark. Which tells us it has to be July 16.
If you enjoyed that mind boggler, here are few more puzzles to test your brain power…
- Mary’s mum has four children. The first child is called April. The second May. The third June. What is the name of the fourth child?
- Two men are playing tennis together. After three full sets, both of them win. How is this possible?
- There are three switches downstairs, each controlling one of the three lightbulbs in the attic. You can turn the switches on and off and leave them in any position, but you are only allowed one trip upstairs. How would you identify which switch controls which light bulb?
- An Arab sheikh tells his two sons to race their camels to a distant city to see who will inherit his fortune. The one whose camel is slower wins. After wandering aimlessly for days, the brothers ask a wise man for guidance. After hearing his advice, they jump on the camels and race to the city as fast as they can. So what advice did the wise man offer to them?
- Isla comes across a lion and a unicorn in a forest. The lion lies every Monday, Tuesday and Wednesday and speaks the truth on the other days. Meanwhile, the unicorn lies on Thursdays, Fridays and Saturdays, but speaks the truth every other day. The lion says: “Yesterday I was lying.” The unicorn says: “So was I.” What day is it?
- A man encountered a bear in a wasteland. There was nobody else there. Both were frightened and ran away. The man ran to the north, the bear to the west. Suddenly the fellow stopped, aimed his gun to the south and shot the bear. What colour was the bear?
- Verity has 12 black socks and 12 white socks in her drawer. Without looking, how many socks must she take from the drawer to be sure she has a matching pair.
- A man lives on the 10th floor and always takes the lift to the ground floor on his way out. When he comes home on a rainy day, or when there are other people in the lift, he goes straight to the 10th floor. Otherwise he gets out on the 7th floor and climbs the stairs to the 10th floor, even if he is in a hurry. Why?
- A writer was 50 years old in the year 2000, but 40 years old in the year 2010. How is this possible?
- A doctor gives his patient five tablets and tells him to take one every 15 minutes. How long will it take him to take all five tablets?
- A cowboy rode into town on Thursday, stayed for four nights and rode out on Thursday. How is this possible?
- A sweet shop lets customers swap five choc bar wrappers for a choc bar. Kevin collects 25 wrappers. How many choc bars can he get?
SCROLL BELOW THE PIC FOR THE ANSWERS….
- Mary. If there are four children and three are April, May and June, the fourth must be Mary herself.
- They are partners playing doubles.
- Keep one switch on for a few minutes. Turn it off, switch on another and go upstairs. Find the warm unlit bulb to identify the one you first switched on. The cold bulb is controlled by the switch you did not use.
- He told them to switch camels. Now they both want to prove the other’s camel is faster.
- Thursday. There is no day when both the lion and unicorn lie. So one is lying today and actually told the truth yesterday. If the lion is lying and told the truth yesterday it would have to be Monday (and yesterday Sunday). However, as both tell the truth on a Sunday that means the unicorn is lying and it told the truth yesterday. That makes today Thursday.
- White. The man must be at the North Pole when he fires and is aiming south whichever way he faces. So the bear is a polar bear.
- Three. Socks don’t come in left and right and there are only two colours, so it takes only three to get a pair in either black or white.
- The man can’t reach the 10th floor button. When it is raining he can use his umbrella to reach it. If there is anyone else he can ask them to help. Otherwise he has to press for the 7th floor and walk the rest.
- He lived in the era BC, not AD. (2010 BC was 10 years earlier than 2000 BC.
- 1 hour. The patient takes one at once, the second after 15 minutes, the third after 30 minutes, the fourth after 45 and the fifth after 1 hour. 11) Thursday was the name of the horse. 12) Six. Kevin can trade in 25 wrappers for five new chocolate bars. Once he eats those, he can trade five wrappers from those bars to get one more.