Question 40139
10m rope = 3/4*pi*10^2
20m rope = 3/4*pi*20^2 + 1/4*pi*5^2
30m rope = 3/4*pi*30^2 + 1/4*pi*15^2 + 1/4*pi*5^2 
40m rope = 3/4*pi*40^2 + 1/4*pi*25^2 + 1/4*pi*15^2

next: If we have to stick with the prescribed answer choices, then 20 ft.
If not, 11.3 m does it.
A = 3/4*pi*r^2
A = 3/4*pi*11.3^2 = 401.1509

Tom Laidlaw