$fun1(\&a, \&b, \&c)$ it's call by reference. Here we will pass the address of a, b, c. After executing this function the values of $a = 10, \: b = 15, \: c = 10$
$fun2(a, b, c)$ it's call by value. After executing $fun2$ the values of $a, b, c$ won't change, because we just pass values to the function. So, the output will be $a + b - 2*c$ is $5$.