How to do it?
Calculate the number of cars on London streets, at a given point in time
# London residents x Avg auto utilisation in a day (%)
Assumptions: First, # of London residents working outside of London = # of non-residents working inside; Second, talking here of cars currently moving (exercise even easier otherwise); Third, traffic constant through the day (otherwise, overweight commute time and severely underweight night time)
Question for you: MBB question, or 2nd tier / boutique? If MBB, above is probably fine and will take you to a roughly right estimation in 2 minutes; if boutique, expect to spend an hour and make 10 thousand calculations to get to precisely wrong answer