the air blown into the balloon, is the content inside the balloon, and therefore the volume of the spherical balloon.
now, if the diameter is 2 feet, the radius of it is half that, or 1 foot.
[tex]\bf \textit{volume of a sphere}\\\\
V=\cfrac{4\pi r^3}{3}\quad
\begin{cases}
r=radius\\
-----\\
r=1
\end{cases}\implies V=\cfrac{4\pi 1^3}{3}\implies V=\cfrac{4\pi }{3}[/tex]
