title: adding a new column based upon values in another column using dplyr
body: <p>I have a column of a data frame <code>df$c_touch</code>:</p>
  
  <pre><code>c_touch
  0
  1
  3
  2
  3
  4
  5
  </code></pre>
  
  <p>Where each number refers to a duration of time, such that <code>0 = 2 mins, 1 = 5 mins, 2 = 10 mins, 3=15 mins, 4=20 mins, 5=30 mins</code>.</p>
  
  <p>I'd like to add another column <code>df$c_duration</code> to be like</p>
  
  <pre><code>c_touch c_duration
  0 2
  1 5
  3 15
  2 10
  3 15
  4 20
  5 30
  </code></pre>
  
  <p>So far I've been using a loop, which is a bit ugly/messy, and I'd rather not use it. Is there a loop-free way of adding the extra column in, particularly using dplyr mutate function (as I'm trying to rewrite all my code using dplyr)?</p>