title: How to divide a dataframe column by 3 but skip division based on another column contents
body: <p>I want to divide both numeric columns by 3 but not the third character column.</p>
  <p>current dataframe:</p>
  <pre><code>col1   col2   col3
  100    10     cat
  200    20     dog
  300    30     NA
  
  desired:
  col1   col2   col3
  10      1     cat
  20      2     dog
  300     30     NA
  </code></pre>
  <p>my current code that isn't based on col3:</p>
  <pre><code>DB &lt;- BD %&gt;% mutate(Col1=Col1/3) %&gt;% mutate(Col2s=Col2/3)
  </code></pre>
  <p>Please help with a solution. Thank you</p>