title: Golf app getting average score by golf course and teebox
body: <p>As you can see, I have 30 golf scorecards. 3 scorecards from course 1 and 27 scorecards from course 2.</p>
  <p>I need a function/method that can calculate the average score for s1,s2,s3…s18, separated by courseID and teeID.</p>
  <p>scorecards</p>
  <pre><code>[
      {courseID: 1,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
      {courseID: 1,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
      {courseID: 1,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
  …3 scorecards
  ],
  
  [
      {courseID: 2,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
      {courseID: 2,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
      {courseID: 2,  teeID: 1   , s1: 4,   s2: 3,   s3: 5,   …s18: 4},
  …27 scorecards
  ];
  </code></pre>