How to calculate mortgage/interest/property tax for rental property?

[Noir Klaus]

We bought 2 homes in 1 lot. We have rented 1 house. It has its own different address. Our loan is for our primary address. So, our mortgage payments are combined for both houses. Let’s say we pay X amount in mortgage.

My question is : how do I calculate mortgage/interest/property tax for other house which we have rented so that I can deduct that from rental income? Also, how do I assign a purchase price for other house?