Let the circumference of the earth be y, the radius be r 1, the original length of the rope be y (just enough for the circumference of the earth), and add 100 meters to make it y+ 100. At this time, the circumference of the circle surrounded by the rope is r2, so

2∏r 1=y

2 * R2 = y+ 100

Then r 1=y/2∏

R2 =(y+ 100)/2∏= y/2 ∏+ 100/2 ∏= r 1+ 100/2∏

So after the rope 100 m, the radius is 100/2∏ m. Even if we take 4, it is still 12.5 meters.