Not sure whether a seasoned consultant is going to blown away by some nervous stuttering newbie fumbling on their shiny new pda, playing with their new cufflinks and showing of their new patek phillipe watch!
To answer the bus question -
Assume there are 300 bus routes in London
Each bus route is about 100 minutes long
Average wait per bus = 10mins
buses travel in both directions
Number of buses = 300*(100/10)*2
Also there are going to be buses being serviced and lying idle in depots so maybe add another 10% to give 6600. (state assumption that almost all buses in London are red)
Or you could answer the question by thinking how many people there are in London, what %age of people use buses, how many people does a bus transport........
Or density - how many stops can you think of in a 1 mile radius from your house? Is this representative of London? How manybuses does this equate to and them scale up to the area of London.