When the number x 1 x x x is multiplied by 417 this gives the answer 9 x x x 0 5 7. Find the missing digits, each of which is represented by an "x" .
Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some. . . .
Think of any three-digit number. Repeat the digits. The 6-digit number that you end up with is divisible by 91. Is this a coincidence?
Three people chose this as a favourite problem. It is the sort of problem that needs thinking time - but once the connection is made it gives access to many similar ideas.
Choose any 3 digits and make a 6 digit number by repeating the 3 digits in the same order (e.g. 594594). Explain why whatever digits you choose the number will always be divisible by 7, 11 and 13.
Using the digits 1 to 9, the number 4396 can be written as the product of two numbers. Can you find the factors?
Gabriel multiplied together some numbers and then erased them. Can you figure out where each number was?
Using the digits 1, 2, 3, 4, 5, 6, 7 and 8, mulitply a two two digit numbers are multiplied to give a four digit number, so that the expression is correct. How many different solutions can you find?
The items in the shopping basket add and multiply to give the same amount. What could their prices be?
Given the products of adjacent cells, can you complete this Sudoku?
Can you find any perfect numbers? Read this article to find out more...
Is there an efficient way to work out how many factors a large number has?
A three digit number abc is always divisible by 7 when 2a+3b+c is divisible by 7. Why?
Play the divisibility game to create numbers in which the first two digits make a number divisible by 2, the first three digits make a number divisible by 3...
Find some triples of whole numbers a, b and c such that a^2 + b^2 + c^2 is a multiple of 4. Is it necessarily the case that a, b and c must all be even? If so, can you explain why?
The number 8888...88M9999...99 is divisible by 7 and it starts with the digit 8 repeated 50 times and ends with the digit 9 repeated 50 times. What is the value of the digit M?
The number 12 = 2^2 × 3 has 6 factors. What is the smallest natural number with exactly 36 factors?
How many integers between 1 and 1200 are NOT multiples of any of the numbers 2, 3 or 5?
Four of these clues are needed to find the chosen number on this grid and four are true but do nothing to help in finding the number. Can you sort out the clues and find the number?
Find the highest power of 11 that will divide into 1000! exactly.
This article takes the reader through divisibility tests and how they work. An article to read with pencil and paper to hand.
Play this game and see if you can figure out the computer's chosen number.
Nine squares are fitted together to form a rectangle. Can you find its dimensions?
Does this 'trick' for calculating multiples of 11 always work? Why or why not?
Which pairs of cogs let the coloured tooth touch every tooth on the other cog? Which pairs do not let this happen? Why?
Becky created a number plumber which multiplies by 5 and subtracts 4. What do you notice about the numbers that it produces? Can you explain your findings?
Explore the relationship between simple linear functions and their graphs.
What is the smallest number of answers you need to reveal in order to work out the missing headers?
Factor track is not a race but a game of skill. The idea is to go round the track in as few moves as possible, keeping to the rules.
Can you work out what size grid you need to read our secret message?
Imagine we have four bags containing numbers from a sequence. What numbers can we make now?
How many different number families can you find?
Each clue in this Sudoku is the product of the two numbers in adjacent cells.
Can you find any two-digit numbers that satisfy all of these statements?
The clues for this Sudoku are the product of the numbers in adjacent squares.
I added together the first 'n' positive integers and found that my answer was a 3 digit number in which all the digits were the same...
6! = 6 x 5 x 4 x 3 x 2 x 1. The highest power of 2 that divides exactly into 6! is 4 since (6!) / (2^4 ) = 45. What is the highest power of two that divides exactly into 100!?
Find the number which has 8 divisors, such that the product of the divisors is 331776.
What is the remainder when 2^2002 is divided by 7? What happens with different powers of 2?
List any 3 numbers. It is always possible to find a subset of adjacent numbers that add up to a multiple of 3. Can you explain why and prove it?
I'm thinking of a number. My number is both a multiple of 5 and a multiple of 6. What could my number be?
Do you know a quick way to check if a number is a multiple of two? How about three, four or six?
Explain why the arithmetic sequence 1, 14, 27, 40, ... contains many terms of the form 222...2 where only the digit 2 appears.
Number problems at primary level that may require resilience.
Find some examples of pairs of numbers such that their sum is a factor of their product. eg. 4 + 12 = 16 and 4 × 12 = 48 and 16 is a factor of 48.
In this problem we are looking at sets of parallel sticks that cross each other. What is the least number of crossings you can make? And the greatest?
This article for teachers describes how number arrays can be a useful representation for many number concepts.
How can you use just one weighing to find out which box contains the lighter ten coins out of the ten boxes?
The puzzle can be solved by finding the values of the unknown digits (all indicated by asterisks) in the squares of the $9\times9$ grid.
The flow chart requires two numbers, M and N. Select several values for M and try to establish what the flow chart does.