The area depends on whether the 8dm path runs top to bottom or left to right
the followin solution will be for left to right
we need to draw a picture (see first attachment)
to simplify it, we can push the paths to the side (see second attachment)
now we can easily see that the usable area is the part not used by the paths
the width is 20-0.7=19.3
the height is 30-0.8=29.2
area=19.3*29.2=563.56 square meters
if it was frmo top to bottom
20-0.8=19.2
30-0.7=29.3
19.2*29.3=562.56 square meters
if the path is like in the attachments, then the usable area is 563.56 square meters
if the 7dm path and the 8dm path are switched, then the usable area is 562.56 square meters