[{Flight:"1",ArrDelayMinutes:"3.0"},{Flight:"1",ArrDelayMinutes:"16.0"},{Flight:"2",ArrDelayMinutes:"15.0"}]

db.collection.aggregate([{"$match":{$expr:{$gt:[{$toDouble:"$ArrDelayMinutes"},15.0]}}},{"$group":{"_id":null,"total":{"$sum":1}}}])