project euler problem 458

Permutations of Project

I do not remember when I solved 3 PE problems in just one day. But it was definitely a long time ago. Just on March 24,  I solved 3 relatively easy PE problems in one day! 

I read the problem for 5 minutes or so before I fully I understood the problem. I was in the mood that I should use 26 letters! But after that the problem is simple and straight forward. The technique was used at least 3 times in previous problems, for example, problem 237, one of my favorite problems. After I set up everything correct, the first shot gave the correct answer.  Do not be misled by 7^7-7!(when did you see PE problem makers tell you how to solve the problem?), if you want to try it in a combinatoric way, you would struggle with the problem in a hard way.