1. Use Math.random function to generate a number between 20 and 100 and then use the ngSwitch directive to display a letter grade based on this class grading policy.
   1. add implements OnInit to the AppComponent class
   2. add variable x in the AppComponent class
   3. add ngOnInit(){ this.x = Math.floor(Math.random()*10);}
   4. add {{x}} in the app.components.html file
   5. You should see numbers from 1-9 when you refresh the page
   6. Change formula in #c to generate numbers from 20 to 100.
   7. Change {{x}} in #d to ngSwitch directive