title: Letter grade function
body: <p>I'm very new to python, I need to write a function that takes a letter grade (a,b,c,d,f, possibly with a + or -) and returns a number grade. The values are 4,3,2,1,0. So an A would be a 4.0, and B would be 3.0. An A+ would increase the number grade value by 0.3 and A- decreases the number grade by 0.3. Upper and lowercase ones are supposed to be able to be used. Some examples.</p>
  
  <pre><code>&gt;&gt;&gt; letter2number('A-')
  3.7
  
  &gt;&gt;&gt; letter2number('B+')
  3.3
  
  &gt;&gt;&gt; letter2number('D')
  1.0
  
  &gt;&gt;&gt; letter2number('a+')
  4.3
  </code></pre>
  
  <p>I do not know where to start with this coding, and I've been on the search everywhere for some helpful code.</p>