title: Randomly choose a value in one column subset by another column
body: <p>What is the simplest formula I can use to randomly choose a value in column A that is associated with a given B value.  So in the table below, I'm looking to randomly choose an A where B = 3.  So I'm randomly choosing between row 1 (5.4) and row 3 (4.2).  Note that this table can be arbitrarily large.</p>
  
  <pre><code>    A     B
  
  1   5.4   3          
  2   2.3   1
  3   4.2   3
  4   9.2   2
      ...   ...
  </code></pre>