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Updated after feedback
Merge pull request #293 from jcarlosgarcia/using-avg

Completed translation of "09-Using_$avg" parts a and b. Resolves #289
Updated after feedback
Updated after feedback
Merge pull request #294 from jcarlosgarcia/using-addtoset

Completed translation of "10-Using_$addToSet" parts a and b. Resolves #290
Merge pull request #299 from jcarlosgarcia/using-push

Completed translation of "11-Using_$push" parts a and b. Resolves #295