A small manufacturing company has been purchasing a casting in its final machined form. The company is now considering whether or not it should buy the rough casting and perform its own machining. The purchase price of the machined part is $4.25 per unit. The price of the rough casting is $1.30 per unit, and it requires $1.80 per unit in labor if the company does its own machining. In addition, if the company does its own machining the fixed costs will increase by $4800 per year. At what annual volume (number of units) is the total cost to purchase equal to the total cost to make?