Fia needs 3/4 cups of brown sugar.
Now she has two types of measuring cups 1/3 measuring cup and 1/8 measuring cup.
Now let us divide 3/4 by each and see which one gives us more accurate answer.
With 1/3 measuring cup
Dividing 3/4 by 1/3, multiplying reciprocal of 1/3 that is 3/1 with 3/4


With 1/8 measuring cup
Dividing 3/4 by 1/8, multiplying reciprocal of 1/8 that is 8/1 with 3/4

=6
Now here we can see that its easier to measure 6 full cups than fraction 9/4 cups.
So we can say that its better to use 1/8 measuring cup.