[{name:"John Doe",City:"OK",Prepaid:"Y"},{name:"Jane Doe",City:"CA",Prepaid:"N"},{name:"Jule Doe",City:"OK",Prepaid:"N"},{name:"Jake Doe",City:"OK",Prepaid:"Y"}]

db.collection.aggregate([{$group:{_id:{city:"$City",prepaid:"$Prepaid"},total:{$sum:1}}},{$group:{_id:"$_id.city",Count:{$push:{k:"$_id.prepaid",v:"$total"}}}},{$project:{_id:0,city:"$_id",Count:{$mergeObjects:[{filter:"prepaid"},{count:{$arrayToObject:"$Count"}}]}}}])