Copy one field value to another within same table
I have a Table- Blood Pressure
Lets assume there are two fields: LeftArm and RightArm
Both fields have a dropdown list of three options each which are exactly the same - lets say "eighty", 'ninety", "hundred".
LeftArm is filled in first followed by RightArm
I want this to happen: If "eighty" is chosen for LeftArm, I want Rightarm to display "eighty" also. This is because both values are usually expected to be the same and will save time for the user. But, I want it to be possible to manually change RightArm to "ninety" or "hundred" if required.
I tried using Look-up but have not managed to get what I want. Please tell me how to do this.