Let be the radius of the Earth

Let be the circumference of the Earth, that is,

Now, originally, the length of the band is . however, it is increased by 20m. now, this increase will lead to a corresponding increase in the radius of the center of the Earth to the band. Let this increase be meters.

Thus we have,

meters

meters.

this is how high the band will be above te ground.

since i am 15 meters tall, i cannot walk under it, can you?

this ingenious solution is due to Soroban, see here

using this method, we didn't even need to know the radius or circumference of the Earth. the method i suggested in the post i directed you to requires it though. that's why Soroban is the genius