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.

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?

Weekly Problem 45 - 2013

Weekly Problem 45 - 2013

Weekly Problem 41 - 2008

Stage: 3 Challenge Level:

How many pairs of numbers of the form $x$, $2x+1$ are there in which both numbers are prime numbers less than $100$

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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