It's 20 metres far away from here.

It's 20 metres far away from here.
It's 20 metres far away from here.

It's 20 metres far away from here.
It's 20 metres away from here.就可以了,不用加far

对的

不对
It's 20 metres far away from here.
应将far去掉。
有了具体路程20 metres 就不能加far了。