I got this question during an interview for a non-consulting job before I learned at all about structuring and break-down analysis tecniques (so I was not prepared at all!). At the time I went down a "number of tube stations X number trains that pass in one day" path and ended up completely lost in a tangle of unnecessary calculations.
I'm curious to know how others would tackle this prolem.
Disclaimer: I'm no expert but this is how I would approach it.
1. Ask for or Estimate(pick a easy number to calc) the number of train routes in London
2. Assume average number of stations in a route - pick number
3. Assume average time between stations - this is also the frequency of trains at a station
4. State that for ease of calculation, you assume that at any point in time there is only train between 2 stations in any direction
5. No of trains = multiply Routes from 1 * (Avg No of stations -1) * Avg time taken between stations * 2
Multiple by 2 because train routes are bi directional
For extra point - say there would be a buffer of 10% for maintenance/peak times
or you can assume a certain percentage of routes are circular so you wont need to mutiply by 2
