title: Mutliply several columns of a dataframe by a factor (scalar)
body: <p>I have a very basic problem and can't find a solution, so sorry in advance for the beginner question.</p>
  
  <p>I have a data frame with several ID columns and 30 numerical columns. I want to multiply all values of those 30 columns with the same factor. I want to keep the the rest of the data frame unchanged. I figured that <code>dplyr</code> and <code>transmute_all</code> or <code>transmute_at</code> are my friends, but I can't find a way to express the function <code>Column1:Column30 * factor</code>. All examples given use simple functions like <code>mean</code> and that doesn't help me with the expression. </p>