Question regarding density and circular motion

I did not understand the answer. The question gives us the density of air, thus we have to calculate the volume of air right - to find the mass.

But what the answer does is that it finds the circular area, sure makes sense, but then it goes and multiplies it with the product of v*t. I do not understand this, isn't 's' found from this equation on the same plane as that of the area of the circle?

It seems that comprehension is a recurring issue I have, I figured it out on my own later that they meant the propeller only and not the circular movement of the entire plane.

Gotta read each question thrice at least I guess 😅

Thank you for this regardless, I would upvote you, but the site won't allow me to upvote you again without upvoting a few others.