You would need 15/4 cups of brown sugar or 3 and 3/4 cups of brown sugar. Since 3/4 a cup is needed for one batch to make 5 batches you need 5 times the amount of brown sugar. To multiply 3/4 by 5 you can rewrite 5 as 5/1 and multiply the numerator of the fractions to get 15 and the denominator to get 4, so the answer is 15/4 which you can rewrite as 3 and 3/4.
