Answer: Yes. The answer is D.
The correct answer would be 4 x 10^4 sq ft.
The decimal representation of any number is a linear combination of powers of 10. In other words, given a number like 123.456, we can expand it as

for any
, so the above is the same as

Similarly, we can write

Now it's a question of reducing the fraction as much as possible. We have
so

10 people are above 80. I hope this helped:)