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 70 pounds of peanuts, the manager decided to mix the 70 pounds of peanuts with some cashews and sell the mixture for $3 50 per pound. How many pounds of cashews should mixed with the peanuts to ensure no change in the revenue?