A nut store normally sells cashews for​ $4.00 per pound and peanuts for​ $1.50 per pound. But at the end of the month the peanuts had not sold​ well, so, in order to sell 40 pounds of​ peanuts, the manager decided to mix the 40 pounds of peanuts with some cashews and sell the mixture for $ 2.00 per pound. How many pounds of cashews should be mixed with the peanuts to ensure no change in the​ revenue?