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db.collection.aggregate([{$unwind:{path:"$answers",includeArrayIndex:"index"}},{$group:{_id:{surveyId:"$surveyId",index:"$index"},A:{$sum:{$cond:[{$eq:["$answers.answer","A"]},1,0]}},B:{$sum:{$cond:[{$eq:["$answers.answer","B"]},1,0]}},C:{$sum:{$cond:[{$eq:["$answers.answer","C"]},1,0]}},all:{$sum:1}}},{$project:{A:{$multiply:[{$divide:["$A","$all"]},100]},B:{$multiply:[{$divide:["$B","$all"]},100]},C:{$multiply:[{$divide:["$C","$all"]},100]},question:{$add:["$_id.index",1]},surveyId:"$_id.surveyId",_id:0}}])