She must buy 50 candies that cost 80 cents per pound must she buy to make a $1 candy mix if she already bought 50 pounds of candy for $1.20 per pound. This problem can be solved by using a simple algebra equation to find the addition candy number which stated as $1 = $1.2*50+$0.8x/(50+x). This equation will result in 0.2x = 10, therefore, x = 50 which represents the additional candy needed.