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db.collection.aggregate([{"$facet":{"category":[{$match:{"_id.category":"furniture"}}],total:[{"$group":{"_id":null,"count":{"$sum":"$annual cost"}}}]}},{$unwind:"$category"},{$unwind:"$total"},{$project:{percentage:{"$multiply":[{"$divide":["$category.annual cost","$total.count"]},100]}}}])