Not drawn to scale

The object can be divided into two separate regions, with different areas. The sum these must equal the total area of the rectangle. The area of the region on the right is:

$(2r)^2=4r^2$ because its width and height are both $2r$

The region on the left has an area of:

$(2r)(48)=96r$

The sum of these must equal 100, so:

$100=4r^2+96r$

$0=4r^2+96r-100$

$0=r^2+24r-25$

$0=(r+25)(r-1)$

r cannot be -25, so it must be 1.

$A=πr^2=π(1)^2=π$

The answer is $\boxed{1}$