Many thanks for your understanding, Mr Tan. Yes, I am new to this app ;)
After seeing your feedback, I have tried to reverse my vote but there is no button to do this reversal act :(

Really appreciate your time to derive an alternative simplier solution for me! Thank you very much and have a good day :)

No worries. Good to note to request to clarify before voting. Have a good week ahead. Note that the primary method used is a simplification of a topic skill named Arithmetric Progression that used to be taught in Sec 3 Additional Mathematics. The dumbed down version of adding a suquence of numbers, is only suitable for cases where you can 1 to the number to get the next term.

Noted Mr Tan. Thanks a lot for your advice :)