First, we will calculate the total daily demand for gas in Paris (left side of the tree). There are about 10 million people living in Paris.
Since people below the age of 18 do not own cars and a lot of people use public transportation in Paris, we can estimate that there is 1 car for every 5 people.
There are also commercial vehicles such as trucks and company cars. (For simplicity’s sake, these are not shown in Diagram 2.) Let’s say that there are 4 times more private vehicles than commercial vehicles.
In total, there are 2.5 million vehicles.
- 20% (0.5m/2.5m) of the total vehicles are commercial vehicles.
- 80% (2m/2.5m) of the total vehicles are private vehicles.
To estimate the consumption per vehicle, we assume that cars have tanks of 50 liters. The tanks must be replenished every 10 days. Thus, each vehicle consumes 5 liters of gas every day.
Since commercial vehicles are on the road more frequently, they consume up to 5 times more gas than private vehicles.
Since this is an interviewer-led case, the interviewer should guide the candidate through the interview.
There are several ways to estimate the number of gas stations. Their feasibility may be influenced by the amount of information the interviewer decides to give the interviewee. (In this case, there is not much information.)
Generally, an estimation method should use values that can be easily estimated and sanity-checked (verified using another approach).