A nut store normally sells cashews for $9.00 per pound and almonds for $4.50 per pound. But at the end of the month the almonds had not sold well, so, in order to sell 60 pounds of almonds, the manager decided to mix the 60 pounds of almonds with some cashews and sell the mixture for $7.75 per pound. How many pounds of cashews should be mixed with the almonds to ensure no change in the revenue?