Excel 2003/2007

```
name, count
a, 3
b, 4
c, 9
...
```

Say I have data like this, is there a built-in function that can automatically count percentage for each row? (ie. a = (3/(3+4+9)) = 18.75%). Also the data will be added continuously. Can I know solutions for both Excel 2003 & 2007. Thanks.