Selling a house from portfolio. How to determine capital gains?

[Tigran Kalaydzhyan]

I am buying a real estate portfolio from another investor. The purchase agreement has only one price for the total portfolio. Now, if I decide to sell one of the properties in the future, how would I determine the capital gains to pay taxes? What is the purchasing price of one house within a portfolio? Should I take <portfolio price>/N, where N is the number of houses?