It depends on “who” you defined as “1 unit” ? Is it Henry or Mary ?
If you define M as “1 unit”, then the working will be like what you mentioned….use total amount, says, $30 minus $3, divided by 2 and add back $3 ( refer to the picture attached ).
But the final answer will still be the same = ( K + 3 ) / 2 , no matter which approach you use.
Hope this answer your doubt.