QP School

Example:

Function Factorial(num As Integer) As Long
    Dim result As Long
    Dim i As Integer

    result = 1
    For i = 1 To num
        result = result * i
    Next i

    Factorial = result
End Function

Sub CalculateFactorial()
    Dim num As Integer

    num = 5 'Enter the number for which you want to calculate the factorial

    MsgBox "The factorial of " & num & " is: " & Factorial(num)
End Sub

Explanation:

This code consists of two parts. 
The first part defines a custom function named Factorial, which calculates the factorial of a given number. 
The second part defines a subroutine named CalculateFactorial, where you can specify the number for which you want to calculate the factorial.

The custom function Factorial is created to calculate the factorial of a number. 
The function takes an integer input num and returns a Long value as the result. 
It uses a For loop to iterate from 1 to num and multiplies the numbers together to find the factorial. 
The CalculateFactorial subroutine calls the Factorial function with a specified number (num) and displays the factorial in a message box.