As soon as you make assumptions outside the stated info the problem becomes unsolvable.
There is no fixed profile of wattage vs. voltage reduction for incandescent bulbs (I am reading the schematic symbol as meaning 'incandescent'). Different wattage rating bulbs have different profiles of net watts consumed versus voltage reduction applied. It is related to how hot the filament is run at rated voltage. Long life incandescents run a cooler filament temp at rated voltage to make them last longer (at lower lumens per watt penalty). Quartz halogen bulbs run very hot filament at rated voltage to achieve higher lumens per watts.
Somewhat irrelavant, but suspeciously coincidential, is the bottom two parallel bulbs wattage summation equals the wattage of the top bulb in series. It makes the circuit do-able with standard off-the-shelf bulbs. (well, maybe not off the shelf in Europe). It is not a good way to actually run bulbs as the lower wattage bulbs will likely get a startup, momentary, overvoltage surge that will reduce their lifespan because the top higher wattage bulb will take longer for its filament to come to operating temp where its effective resistance increases.