Database

[{"_id":{"$oid":"5fe413b097d14d1fcd60bbc9"},"studentName":"Ajay Kumar","standard":"6","subjects":[{"paperName":"Maths","marks":"10"},{"paperName":"English","marks":"20"}]}]

Query

db.collection.aggregate([{$project:{standard:1,marks:{$map:{input:{$filter:{input:"$subjects",cond:{$eq:["$$this.paperName","Maths"]}}},in:{$toInt:"$$this.marks"}}}}},{$unwind:"$marks"},{$group:{_id:"$standard",SubjectsMarks:{$avg:"$marks"}}}])

Result