Construct a function lengthNum so that it will only return the length of a list of numbers. (i.e. it will not work for any other type of list, e.g. a list of String).

Here's a possible implementation of the `lengthNum` function in Python:

def lengthNum(lst):
    """
    Returns the length of a list of numbers.

    Args:
        lst: A list of numbers.

    Returns:
        The length of the list.

    Raises:
        ValueError: If the list contains non-numeric elements.
    """
    for num in lst:
        if not isinstance(num, (int, float)):
            raise ValueError("List contains non-numeric elements")
    return len(lst)

The function takes a list `lst` as input and iterates over its elements to check if they are either integers or floats (using the `isinstance` function). If any element is not a number, it raises a `ValueError`. Otherwise, it returns the length of the list using the built-in `len` function.

Here's an example usage of the function:

>>> nums = [1, 2, 3, 4, 5]
>>> lengthNum(nums)
5

>>> nums = [1, 2, 3, "four", 5]
>>> lengthNum(nums)
ValueError: List contains non-numeric elements