For market sizing questions the following 3 aspects are important:
In general – and I am sure you know that – the result of your approach is much less important than you approach! Hence, use the opportunity to show that you can see the bigger picture but also consider the details and that you are structured in your approach.
Also: Do communicate with your interviewer!
Now, how could you structure your approach using a demand side framework?
Now, in reality you would (and so you should do in an interview) sensitivity and sense check your numbers! In a sensitivity check, you would discuss with the interviewer what you think are the parameters that drove your result, e.g. is it realistic that there are no bikers aged 41-50 that would rent bikes? Probably not, so state again, that this is just a simplification. In a sense check, you would conduct a totally other approach, e.g. a supply side approach or try to get reference numbers from public registers, insurance registers etc.
Remember that the result is much less important than your approach! The interviewer wants to see that you think in a structured way about problems and he wants to be confident that you can easily replicate these results! Therefore, it is important that you communicate with your interviewer and let him participate in your approach. If you are given the problem, take 10 minutes without talking with the interviewer and then state the correct number you will not pass the interview.
I have more market sizing cases available and am happy to coach.
Hope this helps.
Our Goal: #Rental Bikes /Major City
If you are talking about revenue, assuming the cost of a bike is about 100 USD. So the cumulative cost of the bikes is $11M.
Assuming, each bike charge $1/hour, Average riding time is 15 minutes, Bikes are in use for average 20 hours/day and average usage rate is 20%; Revenue = $1*4*20 hours*30 days* 20% =$480/month
So, the revenue from rental bikes in a major city/LA = $480 * 110K bikes = 48000+4800 = $52.8M annually! Again this is cumulative rental bike market in the dollar and cents term!