title: How to specify multiple columns with gather() function to tidy data
body: <p>I want to tidy my data with the gather function but how do I specify multiple columns at once?</p>
  
  <p>Say this is my data:</p>
  
  <pre><code>    Country Country.Code Year X0tot4 X5tot9 X10tot14 X15tot19 X20tot24
  1  Viet Nam          704 1955   4606   2924     2389     2340     2502
  2  Viet Nam          704 1960   5842   4410     2860     2356     2318
  3  Viet Nam          704 1965   6571   5646     4328     2823     2335
  4  Viet Nam          704 1970   7065   6391     5548     4271     2797
  5  Viet Nam          704 1975   7658   6862     6237     5437     4208
  6  Viet Nam          704 1980   7991   7473     6754     6113     5266
  7  Viet Nam          704 1985   8630   7855     7375     6657     6027
  8  Viet Nam          704 1990   9212   8513     7770     7277     6571
  9  Viet Nam          704 1995   9200   9099     8447     7702     7140
  10 Viet Nam          704 2000   7245   9119     9053     8402     7610
  11 Viet Nam          704 2005   6760   7140     8997     8951     8257
  12 Viet Nam          704 2010   7277   6657     7015     8891     8775
  13 Viet Nam          704 2015   7753   7233     6623     6982     8817
  </code></pre>
  
  <p>Now I want to create one new column <code>Age.groups</code> with the variables from <code>X0tot4</code> to <code>X20tot24</code>.</p>
  
  <p>Something like <code>df %&gt;% gather(key = "Age.group", value = c(4:8))</code>. Console says this is not the right column specification. But what is?</p>