What exactly is your question?
I believe it’s correct, but i’m not smart
The area of D is given by:

The average value of f over D is given by:
We have a formula for that( according to the Intersecting chords theorem)

3×4=2×jk
6=jk
She estimated by rounding 907 -> 900 and 626 -> 630 and when you subtract them, you get 270, which is how she estimated >300 people would arrive in the afternoon