The Sea-City shuttle is making the last run of the day. The bus leaves the terminal with some passengers already on board. At the city park, the first stop, the bus picks up the same number of passengers as the number that were on board before it stopped. Seven passengers get off here. At the second stop, the zoo, the bus picks up the same number of passengers as the number that were on board before the bus stopped. Nine passengers get off. At the football stadium, the last stop, no one gets on and 21 passengers get off. The bus returns empty to the terminal. How many passengers were on board when the bus left the terminal?