Made my own simple calculator
Keep It Simple For the User
Rather than making the user type the word Exit it might be better to ask the user
Are you done yet(y/n)? and accept a simple yes/no answer. It also might be better if the answer was not case sensitive so accept Y, y, N and n.

Exit From Main() When Possible
The Main() function may contain clean up code so exiting from the ExitProgram() function may not be the best idea. A second problem I see with the ExitProgram() function is that it is called before the user ever enters a calculation. There are two possible ways to handle this, one would be to move the call to ExitProgram() to after the calculation is performed, the other would be to have a function that contains a do while loop that calls Calc within the loop and tests ExitProgram() in the while condition.

Function Complexity
The function Calc() is overly complex (it does too much) and should be multiple functions. One of the functions should get the user input, and a second function should do the calculation. This would be applying two programming principles, the Single Responsibility Principle and the KISS Principle.

The Single Responsibility Principle states:  


  that every module, class, or function should have responsibility over a single part of the functionality provided by the software, and that responsibility should be entirely encapsulated by that module, class or function.


The Keep It Simple (KISS) Principle is an engineering principle that predates computers, basically it is keep the implementation as simple as possible.