Final answer:
Isabella should buy 6 cups of sugar.
Step-by-step explanation:
To determine how much sugar Isabella should buy, we need to find the total amount of sugar needed for both cookies. For the chocolate chip cookies, Isabella needs 2 3/4 cups of sugar, and for the snickerdoodle cookies, she needs 3 1/5 cups of sugar. Adding these two amounts together, we get 2 3/4 + 3 1/5 = 13/4 + 16/5 = (65 + 64)/20 = 129/20 cups of sugar.
Now we compare the total amount of sugar needed (129/20 cups) to the amounts of sugar that can be bought. Isabella can choose between 5 cups, 6 cups, or 7 cups of sugar. To determine which one she should buy, we need to find the closest amount of sugar to 129/20. By comparing the options, it is clear that 6 cups is the closest amount to 129/20 cups of sugar. Therefore, Isabella should buy 6 cups of sugar.