Can you help explaining these C++ practice problems I am doing? Especially #5.

1- Given the function prototype void fun1(int &, double); is the following function call valid or invalid? Why or why not?

double x, y;

// initialization code of x and y //

fun1(x, y);

2- Given the function prototype void fun1(int &, double); is the following function call valid or invalid? Why or why not?

double x, y;

// initialization code of x and y //

fun1(static_cast(x), 3);

3- Given the function prototype void fun1(int &, double); is the following function call valid or invalid? Why or why not?

int x, y;

// initialization code of x and y //

fun1(x, y);

4- Given the function prototype void fun1(int &, double); is the following function call valid or invalid? Why or why not?

int x, y;

double z;

// initialization code of x, y and z //

fun1(x + y, z);

5- Given the function prototype double fun2(int , double); is the following function call valid or invalid? Why or why not?

double x, y;

// initialization code of x and y //

fun2(x, fun2(x, y));