A whole number can be represented as fraction with any denominator of your choice, but the numerator is the same number as the denominator. For example, 3/3 also equals 1, 4/4 also equals 1, 9/9 = 1, etc. So if you have 2 and 2/3 cups, you must covert the whole number 2, into a fraction with the denominator of 3, if 3/3 equals 1, then 3/3 plus 3/3 is 2. You have 3 cups per whole, and there are two wholes plus 2/3 which is 2 wholes, so Millie needs eight 1/3 cups.