a merchant has coffee worth $10 per pound that she wants to mix with 40 pounds of coffee worth $80 per pound, to get a mixture of coffee that can be…

a merchant has coffee worth $10 per pound that she wants to mix with 40 pounds of coffee worth $80 per pound, to get a mixture of coffee that can be sold for $30 per pound. how many pounds of the $10 coffee should be used?