A number is entered in cell A1 of every sheet in a multi sheet workbook. The task is to sum numbers in cell C2 of the Summary sheet from cell A1 of last x sheets – x is the number of sheets (mentioned in cell B2 of the Summary sheet) from which numbers have to be summed up. So if the number entered in cell B2 of sheet is 3, then sum numbers from cell A1 of last three sheets. Worksheets can get added, removed or renamed.
You may refer to my solution in this workbook.