To start, you need to find your area so that all of your numbers have the same unit of measurement.
You can do this by multiplying the length & the width together. In this case it would be:
84 • 50 = 4200 sq ft
Now, if one standing person occupies 2.5 sq ft all we have to do is divide 4200 sq ft by 2.5 sq ft to figure out how many people can fit in the school gymnasium.
4200 ÷ 2.5 = 1680 people