[/** 1*/{"_id":ObjectId("57e0ed40828913a99c2ceb46"),"time":20,"amount":40},/** 2*/{"_id":ObjectId("57e0ed40828913a99c2ceb47"),"time":40,"amount":20},/** 3*/{"_id":ObjectId("57e0ed40828913a99c2ceb48"),"time":50,"amount":10},/** 4*/{"_id":ObjectId("57e0ed40828913a99c2ceb49"),"time":10,"amount":5}]

db.collection.aggregate([{$match:{time:{$gte:20,$lte:40}}},{$group:{_id:null,amount:{$sum:"$amount"}}},{$project:{_id:0,amount:1}}])