How is the ranking determined ...

[R.E.Boss]

within the number of problems solved?

[rayfil]

within the number of problems solved?

Yes. And for the same number of problems solved, the ranking is according to the date and time when the latest problem was solved.

The previous ranking method (up to about a year ago) was based on a number of point assigned to each problem based on the number of times that problem had been solved and the number of active members. That method was good when the number of problems and the membership was small, since it was somewhat related to the relative difficulty of each problem. With the large number of problems now available and expanding membership, new problems could be carrying a high score for a long time although their relative difficulty may be low. That method was thus abandonned with the advantage of a simplified book keeping for scores.