title: Getting average/mean over a set number of days in R
body: <p>Lets say i have this data frame </p>
  
  <pre><code>Date           DayOfWeek    Url    Hits
  09/01/2016     Thursday     url1   3
  09/01/2016     Thursday     url2   5
  09/01/2016     Thursday     url3   4
  09/02/2016     Friday       url1   7
  09/02/2016     Friday       url3   6
  09/03/2016     Saturday     url2   9
  09/03/2016     Saturday     url1   5
  09/04/2016     Sunday       url2   6
  09/07/2016     Wednesday    url10  4
  09/07/2016     Thursday     url2   3
  09/07/2016     Thursday     url4   2
  09/07/2016     Thursday     url5   3
  09/07/2016     Thursday     url1   3
  09/08/2016     Friday     url1   3
  09/08/2016     Friday     url4   3
  09/08/2016     Friday     url5   2
  09/08/2016     Friday     url8   6
  09/09/2016     Saturday     url2   1
  09/09/2016     Saturday     url3   2
  09/09/2016     Saturday     url5   4
  09/09/2016     Saturday     url1   8
  09/14/2016     Thursday     url1   3
  09/147/2016     Thursday     url2   2
  09/14/2016     Thursday     url3   3
  </code></pre>
  
  <p>I want to get the busiest day of the week in terms of the number of unique url visited. For example, in the data frame there are 3 thursdays, first thursday has 3 unique urls visited, second thursday has 4 and the last thursday has 3... what I plan to do is, sum the number of urls = 3 + 4 + 3/ (# of thursdays = 3) = ave number of unique urls for this day....</p>
  
  <p>For Friday, it would be 2 urls for the first then for the second, there are 4, computation will be 2 + 4 / number of fridays in the data set = 2</p>
  
  <p>I'm trying to resolve this via dplyr. I'm trying to use group_by, but i can't seem to determine the right function combinations to arrive at what I need. </p>