Only 1 cup of sugar will be required to make 16 cookies
As mentioned in the question to make 20 cookies requires 1 1/4 cups of sugar. In order to make 16 cookies the amount of sugar required in the process would be less.
Thus, we can conclude that there exists a direct proportion between the amount of sugar and cookies required to make.
Let us take the amount of sugar required to make 16 cookies as x.
Thus, by applying the unitary method
x = 5/4 × 16/20 (1 1/4 = 5/4)
x = 1
∴ 1 cup of sugar is required to make 16 cookies.