[{"_id":ObjectId("5d3e6bf6a1870026482ae33e"),"ModifiedOn":ISODate("2019-07-29T03:45:58.217Z"),"number_of_employees":NumberInt("100"),"offices":[{"state_code":"NY","country_code":"USA"}]},{"_id":ObjectId("5d3e6bf6a1870026482ae33f"),"ModifiedOn":ISODate("2019-07-29T03:45:58.227Z"),"number_of_employees":NumberInt("100"),"offices":[{"state_code":"NY","country_code":"USA"},{"state_code":"LA","country_code":"USA"}]}]

db.collection.aggregate([{"$match":{"offices.country_code":"USA"}},{"$unwind":"$offices"},{"$project":{"number_of_employees":"$number_of_employees","state_code":"$offices.state_code","_id":0}},{"$group":{"_id":"$state_code","total_employees":{"$sum":"$number_of_employees"}}},{"$project":{"Total_Employees":"$total_employees","State":"$_id","_id":0}}])