last transmission may have truncated the logic....

should look like:

Sub SumSelectedCells()
aRandomNumber = 1
CellsToSum = Selection.Address
LastInRange = Selection.Rows.Count
vSum = WorksheetFunction.Sum(Range(CellsToSum))
Range(CellsToSum)(LastInRange).Offset(0, 2).Value = vSum / vRandomNumber
End Sub