How to call a default parameter in C#

DefaultParameters.cs

//Clafication on default parameters
    public static void Sum(int aNumber, double dNum = 5.5)
    {

        number += aNumber;

        dNumber += dNum;
    }

    //CLARIFICATION
    //To call this method without a parameter and get the default value, you can do
    Sum(10);
    //This will put 10 into int and 5.5 into the other parameter

    //A MISTAKE
    //The problem with this example is that when you call the method using  Sum(10); above, it's an unclear reference to which method to call.
    
    //Does the compiler call?
    public static void Sum(int aNumber, double dNum = 5.5)
    {
        number += aNumber;
        dNumber += dNum;
    }

    //OR will the compiler call this method?
    public static void Sum(int aNumber)
    {
        number += aNumber;
    }

    //This is a mistake on Dr Tiffany's part. She should have renamed the method with a default parameter to something else.
//For example:
    SumWithDefault(int aNumber, double dNum = 5.5)
    {
        number += aNumber;
        dNumber += dNum;
    }

    //Then, we could call this method without ambiguity as such
    SumWithDefault(10);
    //This will place 10 into aNumber and 5.5 into dNum

Thanks