Your question is Expected Visible People. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
If you have an army of 100 people, all of which of different heights, and you ask them to assemble single file in front of you, what is the expected value of the number of people you can see?