Well you need the mixture to start off by equalling $7, no matter what weight it is. The mean of two $6 candies and one $9 candy is $7 (6+6+9 = 21, 21/3 = 7)
So the ratio of 2:1 6$:9$ makes $7
Add 2 and 1 to get 3, 15/3 = 5, and because you're working out how much you need for the $9 candy, 5*1 (Because of the 2:1 ratio) is 5
She needs 5 pounds worth of the 9$ candy
