Irvin Corporation's mixing department began January 2011 with 10,000 gallons of product (40 percent completed) in process. During January, Irvin started 100,000 gallons of new product, of which 15,000 gallons remained in ending inventory (70 percent completed). If materials were added at the beginning of the process, how many equivalent units of production for conversion costs would there be for Irvin using the FIFO costing method?

a. 101,500 units

b. 110,000 units

c. 105,500 units

d. 116,500 units

I know the answer is A but dont know how to calculate