JavaScript Question Response
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Great question!

If you are already familiar with the concept of scope then feel free to jump down to the *Hint* at the end of this explanation!
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Scope
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This problem touches upon a very important topic known as scope.
When you write a program the variables you create are accessible based upon their scope.
Consider the following example:


function A(){
	var message = "This is from function A";
	alert(message);
}


function B(){
	var message;
	messsage = "This is from function B";
	alert(message);
}

function C(){
	var message = "This is from function C";
	function D(){
		alert(message);
	}
	D();
}



Let us first consider functions A and B:

In both A and B we have a variable called message which exists within the scope of the function within which it was created.

Notice in function B I have separated the declaration and the initialization of the variable called message.

It is necessary for us to declare message in function B because the message we created in function A is no longer accesible once we leave A.

If we were to write function B without the line "var message;",
then message would be considered undefined,
and our assignment into that variable during the following line,
	 messsage = "This is from function B";
would fail; we would get an error message in the console log when function B gets used
(which you can check by right clicking on your browser and selecting the Inspect option).

Now let's consider the curious case of function C:
In function C we create another message, and we create another function called D.
Inside D we don't need to declare message because D exists within the same scope as message,
so calling D will alert with the value: "This is from function C".

In terms of the problem you are experiencing:
the variable btnNum is still alive in the scope that your onclick functions are;
this means btnNum's value is right where you left it.

The key to success for this problem will be determining how to create a new scope for each onclick function so that their btnNums live in separate scopes.


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*Hint*
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When you assign a function to a variable, you are assigning the actual text of that function.
In order to send a function whose variables hold values it is important to initizalize the function.
It is possible for a function to return another function.

Happy coding!