Prime numbers

problem
Little Goldbach

Age

11 to 14

Challenge level

How many of the numbers 1 to 20 are not the sum of two primes?
problem
Flow chart

Age

11 to 14

Challenge level

The flow chart requires two numbers, M and N. Select several values for M and try to establish what the flow chart does.
problem
Coins

Age

11 to 14

Challenge level

A man has 5 coins in his pocket. Given the clues, can you work out what the coins are?
problem
Rarity

Age

16 to 18

Challenge level

Show that it is rare for a ratio of ratios to be rational.
problem
Factors and multiples puzzle

Age

11 to 14

Challenge level

Using your knowledge of the properties of numbers, can you fill all the squares on the board?
problem
There's always one isn't there

Age

14 to 16

Challenge level

Take any pair of numbers, say 9 and 14. Take the larger number, fourteen, and count up in 14s. Then divide each of those values by the 9, and look at the remainders.
problem
Numbers as shapes

Age

5 to 7

Challenge level

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

Age

11 to 14

Challenge level

Weekly Problem 24 - 2006
How many of the rearrangements of the digits 1, 3 and 5 give prime numbers?
problem
Different by one

Age

14 to 16

Challenge level

Can you make lines of Cuisenaire rods that differ by 1?
problem
Factor trio

Age

11 to 14

Challenge level

Which of the numbers shown is the product of exactly 3 distinct prime factors?