Teas costing $7 per pound are to be mixed with teas costing $4 per pound to make a 30-pound mixture. If the mixture is to sell for $6 per pound, how many pounds of each kind of tea should be used? A) 12 lb at $7/lb, 18 lb at $4/lb B) 15 lb at $7/lb, 15 lb at $4/lb C)20 lb at $7/lb, 10 lb at $4/lb