>>> A building has 10 floors. At the ground floor, 7 people enter the elevator. At the second floor, 3 people get off and 2 people enter. At the fourth floor, 5 people get off and 2 people enter. At the seventh floor, 1 person gets off and 3 people enter. Finally, at the tenth floor, everyone remaining in the elevator gets off. The question is, How many people were in the elevator when it started its journey, and how many total people exited the elevator by the end of its journey?

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After the first floor, only 7-3+2=6 people are in the elevator.
After the third floor, only 6-5+2=5 people are in the elevator.
After the seventh floor, only 5-1+3=7 people are in the elevator.
Since the elevator is at the tenth floor, there were 7 people in the elevator when it started its journey.
Since 7 people got on and 3+2+5+1+3=14 people got off, the elevator carried a total of 14+7=21 people during its journey.
The answer is 21