The first question asks who sold more rolls. So start with figuring out how many Christie sold.
5 total - 1 2/3 left = 3 1/3 sold
you can convert the numbers to improper fractions with the same denominator. Like this:
5 x (3/3) - (3+2)/3
15/3 - 5/3 = 10/3
10/3 = 3 1/3
So now we know Christie sold more because 3 1/3 dozen is more than 2 1/2 dozen.
The part asks how many more.. Subtract the amounts the two girls sold.
3 1/3 - 2 1/2
10/3 x (2/2) - 5/2 x (3/3)
20/6 - 15/6 = 5/6
Christie sold 5/6 dozen more rolls. A dozen is 12 rolls so if you wanted to go further you just multiply 12 x 5/6 = 10 rolls