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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.

Factor Lines

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


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?

Sieve of Eratosthenes

Age 11 to 14 Challenge Level:
Multiples of 2 are: 2, 4, 6, 8, 10, ...
Multiples of 7 are: 7, 14, 21, 28, 35, ...