Hmmm,
Unfortunately, your formula breaks every internationally agreed maths convention of BIMDAS.
Your formula ignores the importance of the brackets, which must be recognised as the first operation to take place, before any other calculation can take place. Hence the 'B' at the front of BIMDAS.
If you choose to ignore the brackets and start making your own maths laws up by calculating some other step first, you obviously end up with nonsense figures such as you have produced.
They teach BIMDAS in grade 3 or 4 I believe.
If you're happy with breaking maths laws, excellent, go for your life.
As they say, ignorance is bliss.
sigh - re-read my post and my formulas/calculations again.
The maths + operations I used is right, I just used a different value for what you called "duplex". It has nothing to do with brackets.
I assumed the first number was the property number.
i.e. I own 6 properties, so my formula is something like:
1x10, 2x7, 3x6, 4x5, 5x2, 6x1 = 78
but they are all single houses/units, so under your formula it is:
1x10, 1x7, 1x6, 1x5, 1x2, 1x1 = 31
Hence my statement of "my FORMULA is different"...
edit: at the end of the day - I got it wrong - but not due to my maths, just due to mis-reading your initial example and using a different formula.
edit 2: I still like my formula better - it does give credit to people who own multiple properties for longer periods of time...