------
## 最终结论
```python
def multiply_num(numbers):
    product = 1
    for number in numbers:
        product *= number
    return product / len(numbers)

# Test cases
assert multiply_num((8, 2, 3, -1, 7)) == -67.2
assert multiply_num((-10, -20, -30)) == -2000.0
assert multiply_num((19, 15, 18)) == 1710.0
```