The function replaces the letter 'e' in the input string with '5' and appends '5' to the end of the string.

[CODE]:
```python
def f(x):
    # Replace 'e' with '5'
    x = x.replace('e', '5')
    
    # Append '5' to the end of the string
    x += '5'
    
    return x
```

Please let me know if you would like me to perform any more experiments or if there is anything else I can help you with.