Here the circumference is C = 80 inches, and the area is A = 192 in^2.
C = 2pi*r, and A = pi*r^2. We want to find the radius, r, and then the diameter, d.
2pi*r = 80 inches, so pi*r = 40 inches.
192 in^2
A = 192 in^2 = pi*r*r = (40 inches)*r, and so r = --------------- = 4.8 inches.
40 in
The radius of the pie is 4.8 inches, and thus the diameter is 9.6 inches.