Link utilization of sliding window protocol
Link utilitization is about the steady state, i.e., what happens when you send a lot of data... not what happens at startup (at the very beginning of the connection).

Imagine you have a humongous file to send.  It takes far more than 10 frames to send the entire file.  Calculate the total time to send the entire file.  Calculate the total transmission time for that file.  Now divide.

Try doing that calculation again.  I think you'll see that you get a  different answer.  For instance, try calculating what happens if you send a file that is 1000 frames long... or 10,000 frames long.  How long does it take to send the entire file?  What fraction of that was spent in transmission time, as opposed to waiting for acknowledgements?