Using Aggregate function in MongoDB to Divide and Mutliply

If you only are interested in the first array element, use $unwind at the start of the aggregation to convert the array to an object; then you can access the fields using the dot notation.

parktime = 10

results = db.carpark.aggregate([
    {'$unwind':'$rates'},
    {'$project': {
        '_id': 1,
        'ppName': 1,
        'weekdayMin': {
            '$multiply': [
                {'$divide': [
                    parktime,
                    '$rates.weekdayMin'
                ]},
                '$rates.weekdayRate'
            ]
        }
    }
}])

Worked example:

from pymongo import MongoClient

db = MongoClient()['mydatabase']

db.carpark.insert_many([{
  'ppCode': 'A0007',
  'ppName': 'ANGULLIA PARK OFF STREET',
  'parkingSystem': 'Electronic Parking System',
  'vehCat': 'Car',
  'availableLot': 210,
  'parkCapacity': 268,
  'startTime': 700,
  'endTime': 1100,
  'rates': [
    {
      'weekdayMin': 30,
      'weekdayRate': 1.3,
      'satdayMin': 30,
      'satdayRate': 1.3,
      'sunPHMin': 30,
      'sunPHRate': 0.7
    }
  ],
  'modified': '2333'
}, {
  'ppCode': 'Y0019',
  'ppName': 'YAN KIT ROAD / CANTONMENT ROAD OFF ST',
  'parkingSystem': 'Electronic Parking System',
  'vehCat': 'Motorcycle',
  'availableLot': 0,
  'parkCapacity': 8,
  'startTime': 2230,
  'endTime': 700,
  'rates': [
    {
      'weekdayMin': 510,
      'weekdayRate': 0.65,
      'satdayMin': 510,
      'satdayRate': 0.65,
      'sunPHMin': 510,
      'sunPHRate': 0.65
    }
  ],
  'modified': '2333'
}])

parktime = 10

results = db.carpark.aggregate([
    {'$unwind':'$rates'},
    {'$project': {
        '_id': 1,
        'ppName': 1,
        'weekday': {
            '$multiply': [
                {'$divide': [
                    parktime,
                    '$rates.weekdayMin'
                ]},
                '$rates.weekdayRate'
            ]
        }
    }
}])

print('Aggregate:')
for result in results:
   print(result['weekday'])

print('Python:')
results = db.carpark.find()

for result in results:
    for r in result["rates"]:
        weekday = (parktime / r["weekdayMin"]) * r['weekdayRate']
        print(weekday)

Gives:

Aggregate:
0.43333333333333335
0.012745098039215686
Python:
0.43333333333333335
0.012745098039215686