How can I return the sum and average of an int array?

This is the way you should be doing it, and I say this because you are clearly new to C# and should probably try to understand how some basic stuff works!

public int Sum(params int[] customerssalary)
{
   int result = 0;

   for(int i = 0; i < customerssalary.Length; i++)
   {
      result += customerssalary[i];
   }

   return result;
}

with this Sum function, you can use this to calculate the average too...

public decimal Average(params int[] customerssalary)
{
   int sum = Sum(customerssalary);
   decimal result = (decimal)sum / customerssalary.Length;
   return result;
}

the reason for using a decimal type in the second function is because the division can easily return a non-integer result

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Others have provided a Linq alternative which is what I would use myself anyway, but with Linq there is no point in having your own functions anyway. I have made the assumption that you have been asked to implement such functions as a task to demonstrate your understanding of C#, but I could be wrong.

customerssalary.Average();
customerssalary.Sum();