GC53TC6 requests the cacher to find at least 111 mystery caches within a period of 10 days. Bonus points are awarded if these 111 founds do occur within a week and are not "interruped" by finds of a different type.
GC3EFXF does request the cacher to find at least 50 mystery caches within a period of 50 days; this "streak" of 50 finds may be "interrupted" by attending an event or logging a FTF (I'd rather ignore that "FTF"-rule: you may not log an STF in order not to break the mystery challenge streak, and not everyone does log their FTFs in a project-gc-compliant manner). Additionally, not more than 10 finds per cache owner are being counted.
These challenges are not typical streak challenges: if the user does find all of those caches on just one or two days, the user meets the requirements of the challenge and doesn't need to find more caches before or after this to meet the requirements.
