What is the obligatory Zakat amount on a surplus of $3100, given that the rent for two houses is $800 annually and the monthly salary is $200?

Zakat (definition) is obligatory on the rent of the two houses, the saved amount, and the portion of the salary saved, provided a full lunar year (hawl) has passed on it and it reaches the nisab (definition). All of this is added to business assets (if any), and zakat is paid on the total amount once a full year has passed on each asset. It is permissible to pay zakat in advance for money on which a full year has not yet passed. The obligatory amount to be paid is one-quarter of one-tenth (2.5%).