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Strange Numbers

All strange numbers are prime. Every one digit prime number is strange and a number of two or more digits is strange if and only if so are the two numbers obtained from it by omitting either its first or its last digit. Find all strange numbers.

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Factor Lines

Arrange the four number cards on the grid, according to the rules, to make a diagonal, vertical or horizontal line.

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Stars

Can you find a relationship between the number of dots on the circle and the number of steps that will ensure that all points are hit?

Little Goldbach

Stage: 3 Short Challenge Level: Challenge Level:1

Both 4 and 8 can be written as the sum of two prime numbers ($4=2+2$, $8=5+3$). How many numbers less than 20 cannot be written as the sum of two prime numbers?

 

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

 

This problem is taken from the UKMT Mathematical Challenges.
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