Chen is baking muffins and banana bread for a brunch buffet. He needs 3 and one-fifth cups of flour to make the muffins and 3 and two-thirds cups of flour to make the banana bread. Which is the best estimate of the number of cups of flour that Chen needs to bake both recipes? 6 cups of flour 7 cups of flour 8 cups of flour 9 cups of flour

7 cups of flour

reason:

to find how much flour is needed, first you need to combine the two fractions.
to add the fractions, you first have to find a common denominator, which in this case is 15
multiply 1/5 by 3 to get 3/15, and 2/3 by 5 to get 10/15. combined, this is 13/15
since the answer is looking for the best estimate, you take the overall amount of flour, which would be 6 13/15, which is closer to 7 cups than 6

(I may be wrong this is just how I did it)