Question

how to write 30 feet?

Answer 1

If you want to convert 30 to inches it would be 360.

If you want to abbreviate it would be 30 ft.

Hope i helped, If i didn't just tell in the comments so i can tell you the answer with your question. 

Answer 2

If you want to abbreviate it, it would be 30 ft 
If you want to convert it to inches it would be 360 inches
If you want to convert it to yards it would be 10 yards