A retail company purchased a kind of merchandise from three different wholesalers. The total purchasing quantities from each wholesaler are different and certainly the purchasing prices are also different. The total purchasing quantities from the first two wholesalers are given. From the first wholesaler, the retailer purchased 20 items with the unit price of $60/item. From the second wholesaler, the retailer purchased 10 items with the unit price of $55/item. And the purchasing quantity from the third wholesaler is unknown. The unit price paid to the third wholesaler is $50/item. If the retailer sells the merchandise at a unit price of $80/item, then its profit will be 40% of the total cost. What is the quantity bought from the third wholesaler?