Create a new line item to be max of old line item

This might be a silly question, but I just got stuck and couldn't think of a solution. I want to create a new line item showing the max of an existed line items. Take the example below, I want to create a new line item called max days, which takes the max of Days. 

Original:

        Days

A        1

B        2 

C        3

D        4

New:

        Days     Max Days

A        1                4

B        2                4

C        3                4

D        4                4