As the time quantum is q and burst time = r and q<r , so let it runs for q time unit and get switched for s time unit , then again run for q time unit and again gets switched for s time unit... so let it runs n times for q unit so n*q = r
thus n = r/q..
and for (n-1) time there will be switching becoz for the first time there will be no switching
so Total switch overhead = [n-1]*s = [(r/q)-1]*s units