A number N is divisible by 10, 90, 98 and 882 but it is NOT
divisible by 50 or 270 or 686 or 1764. It is also known that N is a
factor of 9261000. What is N?
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.
Prime Order
Age 11 to 14 Short Challenge Level
How many of the three-digit numbers that can be made from all of the digits 1, 3 and 5 (used only once each) are prime?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.