Write a Python function `multiply_and_divide(numbers)` that takes a list of numbers as input. The function should multiply all the numbers in the list together and then divide the result by the length of the list. Return the final result.
#### Example Usage
```python [main.nopy]
print(multiply_and_divide([8, 2, 3, -1, 7])) # Output: -67.2
print(multiply_and_divide([-10, -20, -30])) # Output: -2000.0
print(multiply_and_divide([19, 15, 18])) # Output: 1710.0
```
#### Constraints
- The input list will always contain at least one number.
- The numbers can be integers or floats.