3.

A = IntArr(15, 0, 100);

for(int i =0;i<A.Length;i++) Console.WriteLine($"A[{i+1}] = {A[i]}");

...

public static int[] IntArr(int ArrLen = 6, in int Min = 10, in int Max = 100) {

 int[] Out = new int[ArrLen--];  Random rnd = new Random();

 for (; ArrLen > -1; ArrLen--)

   Out[ArrLen] = rnd.Next(Min, Max);

 return Out;

}

4.

using System.Linq;

...

double[]A = {7, 25, 4.5, 0, 17, 61, -10, 0, 1, -4, 22};

Console.WriteLine($"Sum = {A.Sum()}");