Let's suppose I am starting to manage some money. The money is invested in ETFs, particular de VOO.
Let's suppose I have partner one with 1,000 USD, and with this, I can buy 10 shares of VOO at 100 USD price per share. (not real price). My average price per share would be 100 USD.
Then, another partner 2 gives me 2,400 USD, and I buy more shares of the same ETF, but this time at 120 USD per share. This would update my average price from 100 to 113 per share.
In 3 months, the price of the ETFs will gain 5%, (again, fictional), and the "partner 1" wants to subtract his money at a price of 126 per share.
This means a 5% gain at the price of the partner 2 and 26% gain at the price of the partner one. And more importantly, it means 11.50% gain over the average price of the portafolio.
How should I return (or how much) to the partner one? Over his 26% percent or over 11.5% of the total portfolio?