December 21, 2012
It is sometimes possible, starting with a prime, to add a digit to the front of the number that forms a new prime. For instance, starting from prime 7, adding 1 forms prime 17, adding 3 forms prime 317, adding 6 forms prime 6317, adding 2 forms prime 26317, and so on.
Your task is to write a program to find the largest prime that can be formed in this way. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.