(a) If an authority owns, operates, or provides financial assistance to a multi-family rental housing project, at least twenty percent (20%) of the units in the project shall be set aside for the exclusive use of persons of low income. An authority may group projects being developed concurrently in order to meet the requirement of this subsection.
(b) If an authority provides financial assistance to a multi-family rental housing project, the authority shall establish, as a condition of the assistance, requirements and procedures that insure that all units initially set aside for the exclusive use of persons of low income continue to be so used for at least 15 years after the initial date on which at least fifty percent (50%) of the units in the project are occupied.