"Can I first ask a few questions for clarification? When you say 'impressions,' are you referring to how many people will see it? Also, where is this subway car operating? Is there only one advertisement for the car? May I assume for simplicity that the car was fully emptied at each stop? Where is it located inside the car? Based on the information I have, I'll need to calculate the percentage of people who would have seen this advertisement based on its location inside the subway car each time the car was full and multiply it by how many groups of people would have come in and gone. The number of people inside the car at each stop will depend on the time of day - there will be more during rush hour. If it's packed, people sitting down might not see the ad, so we may have to consider that as well."