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You’d do it the same way you would with any portfolio:

r_port_ = Σw_i_r_i_

where w_i_ = beginning value of stock *i* divided by the beginning value of the subportfolio.

For example, suppose that you have a portfolio with these returns and beginning values:

- Stock A: $10,000,
*r*= 4% - Stock A: $20,000,
*r*= 5% - Stock A: $30,000,
*r*= 2% - Stock A: $40,000,
*r*= −1%

If you want the return for the subportfolio comprising stocks A & C, then the weight on A is ¼ and the weight on C is ¾: $10,000 / ($10,000 + $30,000) and $30,000 / ($10,000 + $30,000), respectively.

The return is ¼(4%) + ¾(2%) = 2.5%.

http://www.aaii.com/journal/article/how-to-calculate-the-return-on-your-portfolio

#WeightedAverageReturn

In excel, it would be sumproduct (returns, dollar balance) / sum(dollar balance)