title: 9's complement of a 4-bit binary number
body: <p>I don't understand how to calculate the 9's complement of a binary number.
  I can apply it to decimal ones, example 15 = (9-1)(9-5) ) 84
  then I thought to proceed with a binary -> decimal -> 9's complement -> binary conversion but I guess it's not the right way to act.</p>
  
  <p><a href="https://i.stack.imgur.com/u8hPn.png" rel="nofollow noreferrer">enter image description here</a></p>