Cubic yards is a measurement of volume, so you want to find the volume of the box. First convert everything to yards: 20 ft by 20 ft by 6 inches is 20/3 yards by 20/3 yards by 6/36 yards. Multiply these together and you get the cubic yards for the box you are going to fill.
You don't mention how deep your sidewalk will be, so I'll assume it will also be six inches. Again, we are looking for the volume, this time of a ring. You can think of it as two discs. Subtract the volume of the inner circle from the volume of the outer circle. Volume is area times depth. The area of a circle is pi (about 3.14) times the radius squared.
Converting your measurements to yards, the radius of the large circle (the outside rim of the sidewalk) is 3 yards, the radius of the small circle (the inside rim of the sidewalk) is 2 yards and the depth is 1/6 of a yard. That means 3.14(3)(3)(1/6) - 3.14(2)(2)(1/6) cubic yards of concrete are required for the sidewalk.
Stephen La Rocque.>