This is one of the unusual time I disagree with TaxMonkey.  The entire building should be 39 years.

§168(e)(2)(A)(i) states the determination is based on if the "building or structure" has rental from dwelling units.  If I'm reading your post correctly, the rental income/value from the "building or structure" is 50% from dwelling units, so it does not qualify as Residential Rental Property.