A department adds materials to a process at the beginning of the process and incurs conversion costs uniformly throughout the process. For the month of January, there were no units in the beginning work in process inventory; $84000$ units were started into production in January; and there were $18000$ units that were $40\%$ complete in the ending work in process inventory at the end of January. What were the equivalent units of production for materials for the month of January?

$84000$ equivalent units

$91200$ equivalent units

$66000$ equivalent units

$76800$ equivalent units