Hey mate u wrote the answer in the same question
There was ninety three thousand one hundred seventy two people
You need to calculate the two zeros of the function. The first zero is the "start", the second zero is the "end" of the trajectory.
Well...
0 = x(-16x+80)
First zero is x=0 (the start of trajectory)
Second zero:
-16x+80=0.... x=5! (The end of the trajectory!)
Good luck!
M.