Database
[{R:[1,10,4,6,20],D:[40,70,90,100,50]}]
Query
db.collection.aggregate([{"$project":{"zip":{"$zip":{"inputs":["$R","$D"]}}}},{"$project":{filtered:{"$filter":{"input":"$zip","as":"z","cond":{"$lt":[{"$arrayElemAt":["$$z",0]},9]}}}}},{"$project":{result:{"$reduce":{"input":"$filtered","initialValue":0,"in":{$add:["$$value",{"$arrayElemAt":["$$this",1]}]}}}}}])