[{"accountNum":1,"credit":2,"rate":50},{"accountNum":1,"credit":5,"rate":10},{"accountNum":2,"credit":3,"rate":50},{"accountNum":3,"credit":2,"rate":50},{"accountNum":3,"credit":5,"rate":30}]

db.collection.aggregate([{$match:{rate:{$ne:0}}},{$addFields:{resultMultiply:{$divide:[{$multiply:["$credit",{"$subtract":[100,"$rate"]}]},100]}}},{$group:{_id:"$accountNum",total:{$sum:"$resultMultiply"}}}])