The cost he purchased it at would be the 1,440 dozen pens * 2.50 equaling $3,600.
The profit he is receiving is calculated by .25 * 12 since there are 12 pens per dozen. This number would be then multiplied by 1,440. So 3 * 1,440= 4,320. This is his profit.
You subtract his cost for purchasing the goods from his profit. This should give you $720 as his profit.