The net investment income tax of Rolly and Leota combined equals to $540.
This refers to the tax that is applicable to income received from investment assets (before taxes) such as bonds, stocks, mutual funds, loans, and other investments.
This net investment income tax was imposed as through the passage of the  Health Care and Education Reconciliation act 2010 and states that the Net investment income tax is liable to 3.80% tax and it applies to individuals have with an NII and MAGI above certain thresholds.
= Net profit from rental income * 3.8%
= $18,000 * 3%
= $540
Therefore, the net investment income tax of Rolly and Leota combined equals to $540.
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