To get any meaningful answer, you have to explain, what you mean by 'last number'!



Is it a number in last row with same value in column A positioned above current row (i.e. determined by position of information rows in table)? When yes, then this means, that this number must be recalculated whenever the table is sorted. At same tame, it means you can use formula to calculate this number;

Or it is a number with same value in column A entered latest of all such rows (i.e. determined by time it was entered). When yes, then there must be a column with entry time or unique row id physically entered into table (which allows to calculate the value in column C using formula), or you must have a procedure which calculates and enters a value into column B whenever a new row is created and value is entered into column A.