$ expr

{ "_id" : 1, "category" : "food", "budget": 400, "spent": 450 }
{ "_id" : 2, "category" : "drinks", "budget": 100, "spent": 150 }
{ "_id" : 3, "category" : "clothes", "budget": 100, "spent": 50 }
{ "_id" : 4, "category" : "misc", "budget": 500, "spent": 300 }
{ "_id" : 5, "category" : "travel", "budget": 200, "spent": 650 }

db.monthlyBudget.find( { $expr: { $gt: [ "$spent" , "$budget" ] } } )

{ "_id" : 1, "category" : "food", "budget" : 400, "spent" : 450 }
{ "_id" : 2, "category" : "drinks", "budget" : 100, "spent" : 150 }
{ "_id" : 5, "category" : "travel", "budget" : 200, "spent" : 650 }

db.supplies.insertMany([
   { "_id" : 1, "item" : "binder", "qty" : NumberInt("100"), "price" : NumberDecimal("12") },
   { "_id" : 2, "item" : "notebook", "qty" : NumberInt("200"), "price" : NumberDecimal("8") },
   { "_id" : 3, "item" : "pencil", "qty" : NumberInt("50"), "price" : NumberDecimal("6") },
   { "_id" : 4, "item" : "eraser", "qty" : NumberInt("150"), "price" : NumberDecimal("3") },
   { "_id" : 5, "item" : "legal pad", "qty" : NumberInt("42"), "price" : NumberDecimal("10") }
])

// Aggregation expression to calculate discounted price

let discountedPrice = {
   $cond: {
      if: { $gte: ["$qty", 100] },
      then: { $multiply: ["$price", NumberDecimal("0.50")] },
      else: { $multiply: ["$price", NumberDecimal("0.75")] }
   }
};

// Query the supplies collection using the aggregation expression

db.supplies.find( { $expr: { $lt:[ discountedPrice,  NumberDecimal("5") ] } });

{ "_id" : 2, "item" : "notebook", "qty": 200 , "price": NumberDecimal("8") }
{ "_id" : 3, "item" : "pencil", "qty": 50 , "price": NumberDecimal("6") }
{ "_id" : 4, "item" : "eraser", "qty": 150 , "price": NumberDecimal("3") }