Iowa State taxes asks,do you have a disabled relative living in your home. I do. How do I figure the amount of my mortgage/utilities that go towards her room and board?

When going through my Iowa State taxes it asks "do you have a disabled relative living in your home". I have gone through the additional info to find that my mother is a qualified relative as far as the state of Iowa is concerned, however, I'm needing guidance as far as how I would figure the "unreimbursed" amount I pay towards her room and board all year. How do I figure this - there's 3 adults (including her) and one child living in the home. My mortgage is $1200, she only pays $200 a month. I pay all utilities, she only pays $20 towards them. How do I use these numbers and household size to figure an amount of my mortgage and utilities that go towards her room and board as far as the State of Iowa in concerned?

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