Database
[{"_id":{"$oid":"5fe413b097d14d1fcd60bbc9"},"studentName":"Ajay Kumar","standard":"6","subjects":[{"paperName":"Maths","marks":"10"},{"paperName":"English","marks":"20"}]}]
Query
db.collection.aggregate([{$unwind:"$subjects"},{$match:{"subjects.paperName":"Maths"}},{$project:{standard:1,marks:{$toInt:"$subjects.marks"}}},{$group:{_id:"$standard",SubjectsMarks:{$avg:"$marks"}}}])