Quick reply on electricity company for PCC

Halo CCI Friends,

I am having problem in Electricity compamy topic "REPLACEMENT OF COMPONENTS"

While caclulating Amount to be capitalised while replacing the process used in some books are as follows-

Expenditure incurred in new plant-                             XXX

less- Current cost of old main-                                    XXX

                                                                                      ________

Additional cost of new main to be capitalised-        XXX

Less- Reuse of old material-                                      XXX

                                                                                      _______

Amount to be capitalised-                                              XXX

the above process is used in some books but my teacher said to calculate like below-

Expenditure incurred-                                                    XXX

Add- Reuse of old material-                                         XXX

                                                                                      ______

Total expenditure-                                                           XXX

Less- Current cost of old material-                              XXX

                                                                                     ________

Amount to be capitalised-                                               XXX

PLZZZZZ SUGGEST ME WHICH METHOD IS CORRECT AND WHAT SHOULD BE FOLLOWED IN THE EXAMINATIONS.....