A radio wave travels 186000 miles per second. 1 mile is equivalent to 1.61 km. 186000 miles is equivalent to 186000*1.61 = 299460 km.
The radio wave travels 299460 km per second. It travels 299460*10^-6 = 0.299460 km in one microsecond.
A radio wave travels 0.299460 km in one microsecond.
