Hi there.
We are going to be comparing 4 cups and 2/3 of a cup (I think; you didn't write the unit of measurement after 2/3).
The first thing we have to do is make sure that our two measurements have the same denominator in their fractions. When we write a whole number - like 4 - we have to remember that the denominator of that whole number is 1 (because there are 4 "1s" in 4).
But there's three thirds in one, so since there's four ones in 4, there's 12 / 3 in four.
So now the two measurements have the same denominator:
12 / 3 : 2 / 3
The denominators are the same - 3 - but the numerators are different: 12 and 2. So now we can ask how many times 12 is bigger than 2. The answer is 6 because 12 / 2 = 6.
So the ratio of flour to brown sugar is 6:1.
