Number Riddles: 30 Math Riddles with Answers and Working on Riddlepedia
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Number Riddles: 30 Math Riddles with Answers and Working

30 riddles · tap Hint or Answer to reveal

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Good number riddles make you think about numbers in a new way: turn a digit upside down, spell a number out, or spot the shortcut a famous mathematician found as a schoolboy. This collection has 30 original and classic number riddles, from place-value puzzles for young kids to logic problems that trip up most adults on the first try.

Every answer includes the working, so you can check it, explain it, or argue about it. Teachers can use the easy section for place value and odd and even numbers, and the medium section for mental math. The tricky section is where the grown-ups come in.

Easy Number Riddles for Kids

I’m the number of legs on a spider, the arms on an octopus and the sides of a stop sign. What number am I?

Hint
A stop sign is an octagon.
Answer
  1. A spider has 8 legs, an octopus 8 arms, and an octagon 8 sides.

I’m an odd number. Take away one letter and I become even. What number am I?

Hint
Spell the numbers from one to ten and look inside each word.
Answer
Seven. Take away the S and you’re left with “even.”

I’m a two-digit number. My tens digit is 3, and my ones digit is double my tens digit. What number am I?

Hint
Work out the ones digit first.
Answer
  1. Tens digit 3, ones digit 3 × 2 = 6.

I’m the biggest two-digit number. Add just 1 to me and I suddenly need a third digit. What number am I?

Hint
Both of my digits are the biggest digit there is.
Answer
  1. 99 + 1 = 100.

I’m between 10 and 20. I’m odd, and my two digits add up to 4. What number am I?

Hint
List the odd numbers from 11 to 19 and add their digits.
Answer
  1. 1 + 3 = 4 (11 gives 2, 15 gives 6, 17 gives 8, 19 gives 10).

Turn me upside down and I become 3 smaller. What digit am I?

Hint
Only a few digits still look like a digit when you flip them.
Answer
  1. Upside down, 9 becomes 6, and 9 − 6 = 3.

I’m a counting number. Add me to myself or multiply me by myself, and you get the same answer. What number am I?

Hint
Try the small numbers, one at a time.
Answer
  1. 2 + 2 = 4 and 2 × 2 = 4.

If you add two odd numbers together, is the answer odd or even?

Hint
Try 3 + 5, then 7 + 9.
Answer
Always even. 3 + 5 = 8 and 7 + 9 = 16. The two “leftover” ones make a pair.

I’m a three-digit number. My hundreds digit is 1 more than my tens digit, and my tens digit is 1 more than my ones digit. My digits add up to 12. What number am I?

Hint
Three digits in a row always add up to three times the middle one.
Answer
  1. 5 + 4 + 3 = 12, and each digit is 1 less than the one before it.

How many times can you subtract 5 from 25?

Hint
Read the question very carefully. What’s left after the first time?
Answer
Once. After that, you’re subtracting 5 from 20, not from 25.

Medium Number Riddles

When you spell me out in English, I have exactly as many letters as my value. What number am I?

Hint
I’m less than ten.
Answer
Four. F-O-U-R has 4 letters. No other number word matches: one, two, six and ten have 3 letters; five and nine have 4; three, seven and eight have 5.

Three different positive whole numbers give the same answer whether you add them or multiply them. What are they?

Hint
The smallest of them is 1.
Answer
1, 2 and 3. 1 + 2 + 3 = 6 and 1 × 2 × 3 = 6.

Think of any number. Double it, add 34, halve the result, then subtract the number you started with. What’s your answer?

Hint
Try it with two different starting numbers and compare.
Answer
17, every time. (2n + 34) ÷ 2 = n + 17, and n + 17 − n = 17.

If you write every whole number from 1 to 100, how many times do you write the digit 9?

Hint
Count the 9s in the ones place and in the tens place separately.
Answer
  1. Ones place: 9, 19, 29 … 99 is 10. Tens place: 90 to 99 is 10 more.

My grandma’s age is both a square number and a cube number. She’s older than 1 and younger than 100. How old is she?

Hint
A square number is a number times itself; a cube is a number times itself times itself.
Answer
  1. 8 × 8 = 64 and 4 × 4 × 4 = 64.

I’m a two-digit number whose digits add up to 7. Swap my digits and the new number is 45 bigger than me. What number am I?

Hint
For the new number to be bigger, my ones digit has to be the bigger one.
Answer
  1. 61 − 16 = 45, and 1 + 6 = 7.

I’m the only number whose letters, when spelled in English, are in alphabetical order. What number am I?

Hint
I’m a multiple of ten.
Answer
Forty. F, O, R, T, Y are in alphabetical order.

Which is more: six dozen dozen, or half a dozen dozen?

Hint
A dozen is 12. Say each phrase slowly.
Answer
Six dozen dozen. 6 × 12 × 12 = 864, but half a dozen dozen is 6 × 12 = 72.

I’m a three-digit even number. I read the same forwards and backwards, and my digits add up to 5. What number am I?

Hint
My first and last digits are the same, and the last digit of an even number is even.
Answer
  1. 2 + 1 + 2 = 5, and it ends in 2, so it’s even.

A plate holds 30 cookies. At lunch, half of them are eaten. At snack time, a third of the cookies that are left are eaten. How many cookies are still on the plate?

Hint
Take the third from what’s left, not from 30.
Answer
  1. 30 ÷ 2 = 15 left; 15 ÷ 3 = 5 eaten; 15 − 5 = 10.

Multiply together every number on a phone’s keypad. What’s the answer?

Hint
Look closely at the bottom row of the keypad.
Answer
  1. The keypad has a 0 on it, and anything multiplied by 0 is 0.

Tricky Number Riddles for Grown-Ups

A bat and a ball cost $1.10 together. The bat costs $1.00 more than the ball. How much does the ball cost?

Hint
It isn’t 10 cents. Check your answer by adding it up.
Answer
5 cents. The bat is $1.05, and $1.05 + $0.05 = $1.10.

How many squares of any size are there on a chessboard?

Hint
It’s a lot more than 64. Count the 2 × 2 squares, the 3 × 3 squares, and so on.
Answer
  1. 64 + 49 + 36 + 25 + 16 + 9 + 4 + 1 = 204.

Six friends meet, and everyone shakes hands with everyone else exactly once. How many handshakes are there?

Hint
Each person shakes 5 hands, but every handshake involves two people.
Answer
  1. 6 × 5 = 30, divided by 2 because each handshake was counted twice.

What comes next: 1, 11, 21, 1211, 111221, ?

Hint
Read each term aloud and describe what you see.
Answer
  1. The term 111221 is “three 1s, two 2s, one 1.”

The pages of a book are numbered from 1, and it takes 192 digits to number them all. How many pages does the book have?

Hint
Pages 1 to 9 use one digit each, and pages 10 to 99 use two.
Answer
  1. Pages 1-9 use 9 digits, pages 10-99 use 180, which makes 189; 3 more digits is one three-digit page, page 100.

Multiply together every whole number from 1 to 10: 1 × 2 × 3 × … × 10. How many zeros are at the end of the answer?

Hint
Every zero at the end needs a 2 and a 5 to pair up.
Answer
  1. The answer is 3,628,800. Only 5 and 10 bring a factor of 5, and there are plenty of 2s to pair with them, so you get exactly two zeros.

A girl says, “The day before yesterday I was 10. Next year I’ll turn 13.” She’s telling the truth. What is today’s date, and when is her birthday?

Hint
She needs three birthdays between the day before yesterday and the end of next year.
Answer
Today is January 1st, and her birthday is December 31st. She was 10 on December 30, turned 11 on December 31, turns 12 this December and 13 next December.

Starting at exactly 12 noon and counting noon itself, how many times do the hour and minute hands of a clock lie exactly on top of each other before midnight?

Hint
The minute hand keeps lapping the hour hand. How many laps does it gain in 12 hours?
Answer
  1. The minute hand goes around 12 times while the hour hand goes around once, so it gains 11 laps. The hands meet every 12/11 of an hour (about every 1 hour 5 minutes 27 seconds): at 12:00, about 1:05, 2:11 … up to about 10:54.

A schoolboy was told to add up all the numbers from 1 to 100, and he did it in moments. What’s the total?

Hint
Pair the first number with the last one: 1 + 100, then 2 + 99…
Answer
  1. There are 50 pairs that each add up to 101, and 50 × 101 = 5050. The boy, the story goes, was Carl Friedrich Gauss.

Make It a Game

  • Show your working: in class, give points for the answer and a bonus point for explaining the working out loud. The explanation often teaches more than the answer.
  • Number riddle relay: split into teams. Each team solves one riddle and then writes a new riddle of the same kind (for example, “I’m a two-digit number…”), which goes to the next team.
  • Hint auction: for older kids and adults, a correct answer without the hint scores three points, and each hint used costs one.
  • Riddle of the day: put one riddle on the board each morning and reveal the answer (with the working) at the end of the day.

For more brain work, try our brain-teasing riddles, our riddles for older kids or our classroom riddles.