title: Using R to manipulate excel spreadsheet data and return output to seperate sheet
body: <p>So I have this excel spreadsheet called <code>'Birth.xls'</code>.
  and I have 10 sheets inside this single spreadsheet in this format:</p>
  
  <pre><code>Sheet1
  Number  Value 
  3   0.125
  6   0.2
  9   0.275
  12  0.35
  15  0.425
  17  0.5
  19  0.575
  21  0.65
  23  0.725
  25  0.8
  27  0.875
  29  0.95
  31  1.025
  Sheet2
  Number  Value
  3   0.614
  6   0.654
  9   0.694
  12  0.734
  15  0.774
  17  0.814
  19  0.854
  21  0.894
  23  0.934
  25  0.974
  27  1.014
  29  1.054
  31  1.094
  </code></pre>
  
  <p>How would I create another 'sheet' in this spreadsheet that has the row averages of the sheets I have given above using R?...I understand that I could do this in excel but since I have 10 sheets or more sometimes, it can be quite a pain...especially when I have to do more complicated things than averages....So all in all, I would want an output similar to the one below</p>
  
  <pre><code>Sheet3
  Number  Average Value
  3   0.3695
  6   0.427
  9   0.4845
  12  0.542
  15  0.5995
  17  0.657
  19  0.7145
  21  0.772
  23  0.8295
  25  0.887
  27  0.9445
  29  1.002
  31  1.0595
  </code></pre>