Wednesday 10 June 2015

Selected Math Riddles - Set 1 (with solutions)

1) If 1/2 of 5 is 3, then what is 1/3 of 10?


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2) Does a pound of gold or a pound of feathers weight more?


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3) If you randomly choose one of the following answers to this question, what is your chance of getting it right?
a) 50%
b) 25%
c) 0%
d) 75%


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4) A claustrophobic person gets on a train. The train enters a tunnel just as it is leaving the station. Where is the best place for him to sit?


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5) Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a sports car; behind all of the others is bicycles. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 2, revealing a bicycle. He then says to you, "Do you want to pick door No. 3?"
Is it to your advantage to switch your choice?


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6) A homeless beggar picks up cigarette butts from the ground and can make a cigarette with 4 butts. If he finds 16 cigarette butts, how many cigarettes can he make?


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7) Little Raju is walking home. He has ₹300 he has to bring home to his mom. While he is walking a man stops him and gives him a chance to double his money. The man says "I'll give you ₹600 if you can roll 1 die and get a 4 or above, you can roll 2 dice and get a 5 or 6 on at least one of them, or you can roll 3 dice and get a 6 on at least on die. If you don't I get your ₹300."
What does Raju do to have the best chance of getting home with the money?


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8) Ram and Krishna are in class when their teacher gives Ram the sum of two numbers and Krishna the product of the same two numbers (these numbers are greater than or equal to 2). They must figure out the two numbers.
Ram:           I don't know what the numbers are.
Krishna:      I knew you didn't know the numbers... But neither do I.
Ram:           In that case, I do know the numbers now.
What are the numbers?


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9) What is the next number in the sequence?
1        11      21     1211          111221       312211


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10) There are several books on a bookshelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?


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11) There are 100 coins scattered in a dark room. 90 have heads facing up and 10 are facing tails up. You cannot tell which coins are which. How do you sort the coins into two piles that contain the same number of tails up coins?


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12) A rat is placed at the beginning of a maze and must make it to the end. There are four paths at the start that he has an equal chance of taking: path A takes 5 minutes and leads to the end, path B takes 8 minutes and leads to the start, path C takes 3 minutes and leads to the end, and path D takes 2 minutes and leads to the start.
What is the expected amount of time it will take for the rat to finish the maze?


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13) You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not?
(Hint: The glass is a right cylinder)


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14) Does a ton of feathers or a ton of bricks weigh more?


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15) Mr. Lalith has 4 daughters. Each of his daughters has a brother. How many children does Mr. Lalith have?


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16) A tomato plant is 3 meters long. The bottom foot of the plant doesn't grow any tomatoes but the rest of the plant grows a tomato every 5 inches. How many vegetables can grow off of the plant?


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17) Ajay has missed an excessive number of days of school, so he must meet with Principal Rajeev. Mr. Rajeev asks him "Why on Earth have you missed so many days?"
Ajay replies "There just isn't enough time for school. I need 8 hours of sleep a day, which adds up to about 122 days a year. Weekends off is 104 days a year. Summer vacation is about 60 days. If I spend about an hour on each meal, that's 3 hours a day or 45 days a year. I need at least 2 hours of exercise and relaxation time each day to stay physically and mentally fit, adding another 30 days.
Add all of that up and you get about 361 days. That only leaves 4 days for school."
The principal knows Ajay is full of it, but can't figure out why. Where is Ajay going wrong?


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18) A deaf and mute man goes to a railway station. Tickets for the train are 50 rupees each. The man goes to the ticket booth and hands the man inside just a hundred rupees. The man in the booth hands him two tickets.
How did the man in the booth know to give him two tickets without even looking at him?


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19) Two cities Delhi and Chennai are exactly 2000 km apart. Praveen leaves City Delhi driving at 50 km/hr and Avinash leaves City Chennai a half an hour later driving 90 km/hr. Who will be closer to City Chennai when they meet?


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20) How long is the answer to this question.


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