WeighedRandom

// Random system class
Random ran = new Random();

// Array of ranges to enter for random max index
int[] ListOfMaxRanges = new int[] {5,5,5,5,5,5,5,5,5,5,10,10,10,10,20,20,20,50,50,100};
// get a random MaxIndex number from the array ListOfMaxRanges
int MaxRange = ListOfMaxRanges[ran.Next(ListOfMaxRanges.Length)];
// Insert my max range to get final number
int MyWeighedRandomNumber = ran.Next(MaxRange);