Database
[{"analysis":[{"0":0.36965591924860347},{"5":0.10391287134268598},{"10":0.086884394}]}]
Query
db.collection.aggregate([{"$addFields":{"average":{"$multiply":[{"$divide":[{"$reduce":{"input":"$analysis","initialValue":0,"in":{"$let":{"vars":{"sum":"$$value","data":"$$this"},"in":{"$add":["$$sum",{"$arrayElemAt":[{"$arrayElemAt":[{"$map":{"input":{"$objectToArray":"$$data"},"as":"m","in":["$$m.k","$$m.v"]}},0]},1]}]}}}}},{"$size":"$analysis"}]},100]}}}])