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12 Math Riddles Only the Smartest Can Get Right

You might want to break out a calculator because it takes a genius to solve these tricky math riddles.

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House numbers

My twin lives at the reverse of my house number. The difference between our house numbers ends in two. What are the lowest possible numbers of our house?

Old retro weathered cast iron plate with number 19 closeupAlexander Varbenov/Shutterstock


These math riddles aren’t easy to solve. Do you think you know the answer to this one? The lowest possible numbers for our house are 19 and 91.

Woman Holding Wire Basket of Fresh Chicken Eggs HorizontalSarah_Baird/Shutterstock

Egg equation

If a hen and a half lay an egg and a half in a day and a half, how many eggs will half a dozen hens lay in half a dozen days?

A lot of brown eggsMaggie_ray/Shutterstock


Two dozen. If you increase both the number of hens and the amount of time available four-fold, the number of eggs increases 16 times. 16 x 1.5 = 24. If math riddles aren’t your thing, try solving these tricky detective riddles.

cardsAaron Amat/Shutterstock

Card question

A small number of cards has been lost from a complete pack. If I deal among four people, three cards remain. If I deal among three people, two remain and if I deal among five people, two cards remain. How many cards are there?

Pile of poker card texture. Casino cards as background.Jenov Jenovallen/Shutterstock


There are 47 cards.

If your favorite riddles don’t involve arithmetic, try solving these riddles for kids!

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Knight moves

I have a calculator that can display ten digits. How many different ten-digit numbers can I type using just the 0-9 keys once each, and moving from one keypress to the next using the knight’s move in chess? (In chess, the knight move in an L-shape – one square up and two across, two squares down and one across, two squares up and one across, and other like combinations)

Black knight chess piece on the board with blue background.Min C. Chiu/Shutterstock


You can form the numbers 5034927618 and 5038167294. You can also form their reverses: 8167294305 and 4927618305. Hence four different numbers can be made. The key point is to realize that the number must start or end on the ‘5’ key, followed/preceded by the ‘0’ key, otherwise, there is no way of using all ten keys during the route. Only 2 percent of people can solve Einstein’s Riddle. Can you?

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Two by two

You know 2 + 2 comes to the same as 2 x 2. Now find a set of three different whole numbers whose sum is equal to their total when multiplied.

 This is a num pad of calculatorJaree/Shutterstock


The three different whole numbers whose sum is equal to their total when multiplied are 1, 2, and 3.

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Apple harvest

Mrs. Jones was very proud of her apple tree. One autumn, after harvesting her apples, she called her three sons together. “Here are 150 apples,” she said. “I want you to take them to the market tomorrow and sell them for me.” She gave Paul 15 apples, Nick 50, and Ben 85. “Your job,” added Mrs. Jones, “is to sell the apples in such a way that each of you brings home the same amount of money.” How do they do it?

Fresh organic red apples in orchardvvvita/Shutterstock


The first buyer purchases 12 dozen apples at $1 per dozen. Paul sells him one dozen and has three apples left; Nick sells him four-dozen and has two apples left; and Ben sells him seven-dozen and has one apple left. Then a second buyer comes along and buys all their remaining apples for $3 apiece. The three brothers head home with $10 each.

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Seven times

What is the smallest whole number that is equal to seven times the sum of its digits?

Close up of a pencil eraser isolated on white.Nik Bruining/Shutterstock


The answer to this math riddle is 21. You probably just guessed to answer this math riddle, which is fine, but you can also work it out algebraically. The two-digit number ab stands for 10a + b since the first digit represents 10s and the second represents units. If 10a + b = 7(a + b), then 10a + b = 7a + 7b, and so 3a = 6b, or, more simply, a = 2b. That is, the second digit must be twice the first. The smallest such number is 21.

Part of ten dollar bill �¢?? American money.YamabikaY/Shutterstock

Lunch money

John noticed that the amount he was paying for his lunch was a rearrangement of the digits of the amount of money he had in his pocket, and that the money he had left over was yet another rearrangement of the same three digits! How much money did John start with?

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John started with $9.54. The money can be written with just three digits—so it must be between $1.01 and $9.99. Trial and error shows that there is only one set of numbers that fit this question: $9.54 = $4.59 + $4.95. If you can solve these math riddles easily, you’ll want to try your hand at what an MIT professor called the “hardest logic puzzle ever.”

Group of three young horses on the pastureGrigorita Ko/Shutterstock

Animal riddle

In reply to an inquiry about the animals on his farm, the farmer says: “I only ever keep sheep, goats, and horses. In fact, at the moment they are all sheep bar three, all goats bar four, and all horses bar five.” How many does he have of each animal?

sheep farmStasis Photo/Shutterstock


The farmer has 3 sheep, 2 goats, 1 horse. Adding 3, 4, and 5, in this case, gives twice the number of animals, so there must be six animals altogether. Here are some genius math tricks you’ll wish you’d known all along.

business, finance, investment, saving and corruption concept - close up of dollar cash money and piggy bank on tableSyda Productions/Shutterstock


Old Granny Adams left half her money to her granddaughter and half that amount to her grandson. She left a sixth to her brother, and the remainder, $1,000, to the dogs’ home. How much did she leave altogether?

business, finance, investment, saving and corruption concept - close up of dollar cash money and piggy bank on tableSyda Productions/Shutterstock


She left $12,000. One half plus one quarter plus one-sixth equals eleven-twelfths. So, the remainder, $1,000, is one-twelfth of the whole, which must have been $12,000.

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Brother puzzle

One brother says of his younger brother: “Two years ago, I was three times as old as my brother was. In three years’ time, I will be twice as old as my brother.” How old are they each now?

Close-up of a pile of uncompleted elements of a white puzzle. A huge number of rectangular pieces from one large white mosaicMehaniq/Shutterstock


The elder is 17, the younger 7. Two years ago, they were 15 and 5 respectively, and in three years’ time, they will be 20 and 10. Can you pass this elementary school math test?

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Upside down numbers

What is the smallest number that increases by 12 when it is flipped and turned upside down?

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The answer is 86. When it is turned upside down and flipped, it becomes 98, which is 12 more than 86. If you were able to solve all of these math riddles, try your hand at some of the hardest riddles ever written.

Morgan Cutolo
Morgan is an Associate Editor at Reader’s Digest. She graduated from the University of New Hampshire in 2016 where she received her Bachelor of Arts in Journalism. She writes for, helps lead the editorial relationship with our partners, manages our year-round interns, and keeps the hundreds of pieces of content our team produces every month organized. In her free time, she likes exploring the seacoast of Maine where she lives and works remotely full time and snuggling up on the couch with her corgi, Eggo, to watch HGTV or The Office.