Complete the function so that it finds the mean of the three scores passed to it and returns the letter value associated with that grade.
| Numerical Score | Letter Grade |
|---|---|
| 90 <= score <= 100 | 'A' |
| 80 <= score < 90 | 'B' |
| 70 <= score < 80 | 'C' |
| 60 <= score < 70 | 'D' |
| 0 <= score < 60 | 'F' |
Tested values are all between 0 and 100. There's is no need to check for negative values or values greater than 100.