If you have ten counters numbered 1 to 10, how many can you put into pairs that add to 10? Which ones do you have to leave out? Why?

Investigate the smallest number of moves it takes to turn these mats upside-down if you can only turn exactly three at a time.

Place the numbers from 1 to 9 in the squares below so that the difference between joined squares is odd. How many different ways can you do this?

How many legs do each of these creatures have? How many pairs is that?

Is it possible to place 2 counters on the 3 by 3 grid so that there is an even number of counters in every row and every column? How about if you have 3 counters or 4 counters or....?

What do you see as you watch this video? Can you create a similar video for the number 12?

In how many different ways can you break up a stick of 7 interlocking cubes? Now try with a stick of 8 cubes and a stick of 6 cubes.

Think of a number, square it and subtract your starting number. Is the number you’re left with odd or even? How do the images help to explain this?

Daisy and Akram were making number patterns. Daisy was using beads that looked like flowers and Akram was using cube bricks. First they were counting in twos.

This problem challenges you to find out how many odd numbers there are between pairs of numbers. Can you find a pair of numbers that has four odds between them?

How would you create the largest possible two-digit even number from the digit I've given you and one of your choice?

Can you order the digits from 1-3 to make a number which is divisible by 3 so when the last digit is removed it becomes a 2-figure number divisible by 2, and so on?

In this calculation, the box represents a missing digit. What could the digit be? What would the solution be in each case?

Are these statements always true, sometimes true or never true?

Look at what happens when you take a number, square it and subtract your answer. What kind of number do you get? Can you prove it?

This article introduces the idea of generic proof for younger children and illustrates how one example can offer a proof of a general result through unpacking its underlying structure.

You have 4 red and 5 blue counters. How many ways can they be placed on a 3 by 3 grid so that all the rows columns and diagonals have an even number of red counters?

Arrange the numbers 1 to 6 in each set of circles below. The sum of each side of the triangle should equal the number in its centre.

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?

Are these statements relating to odd and even numbers always true, sometimes true or never true?

Can you put the numbers 1-5 in the V shape so that both 'arms' have the same total?

In this game for two players, the idea is to take it in turns to choose 1, 3, 5 or 7. The winner is the first to make the total 37.

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?

This investigates one particular property of number by looking closely at an example of adding two odd numbers together.

Mr Gilderdale is playing a game with his class. What rule might he have chosen? How would you test your idea?

What happens when you add three numbers together? Will your answer be odd or even? How do you know?

This problem looks at how one example of your choice can show something about the general structure of multiplication.

If there are 3 squares in the ring, can you place three different numbers in them so that their differences are odd? Try with different numbers of squares around the ring. What do you notice?

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?

Choose four of the numbers from 1 to 9 to put in the squares so that the differences between joined squares are odd.

There is a clock-face where the numbers have become all mixed up. Can you find out where all the numbers have got to from these ten statements?

Try grouping the dominoes in the ways described. Are there any left over each time? Can you explain why?

There are ten children in Becky's group. Can you find a set of numbers for each of them? Are there any other sets?

I am less than 25. My ones digit is twice my tens digit. My digits add up to an even number.

Try to stop your opponent from being able to split the piles of counters into unequal numbers. Can you find a strategy?

How many different sets of numbers with at least four members can you find in the numbers in this box?

Use cubes to continue making the numbers from 7 to 20. Are they sticks, rectangles or squares?

Can you find the chosen number from the grid using the clues?

Can you sort numbers into sets? Can you give each set a name?

Ben’s class were cutting up number tracks. First they cut them into twos and added up the numbers on each piece. What patterns could they see?

Use the interactivities to fill in these Carroll diagrams. How do you know where to place the numbers?

This article for teachers describes how number arrays can be a useful representation for many number concepts.

Florence, Ethan and Alma have each added together two 'next-door' numbers. What is the same about their answers?

This activity is best done with a whole class or in a large group. Can you match the cards? What happens when you add pairs of the numbers together?

You can trace over all of the diagonals of a pentagon without lifting your pencil and without going over any more than once. Can the same thing be done with a hexagon or with a heptagon?