Disciplined Boy/Girl
Assume that you're late for the school and when you reached there, you found that all of your 20 classmates are standing in a row. Then you have to occupy a position in that row, how many options are there for you?
Clarification: There is exactly one position vacant in front of and behind your each classmate.
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1 solution
You can stand either in the front of the row (1 position) or behind each of your 20 classmates (20 positions).
Therefore, you have 20 + 1 = 21 options altogether.