Final answer:
One serving of the described food includes 17 grams of total sugar, which is the sum of naturally occurring and added sugars.
Step-by-step explanation:
When a food label indicates that there are 13 grams of sugar, 4 grams of added sugar, and 15 grams of carbohydrates per serving, the total grams of sugar per serving are the grams of sugar already present.
The label specifies the presence of naturally occurring sugars and added sugars separately, yet both types of sugar contribute to the total sugar content.
Since the label does not specify that the 13 grams of sugar includes the 4 grams of added sugar, we would add these two amounts to find the total quantity of sugar.
Therefore, one serving of this food includes 17 grams of total sugar (13 grams of naturally occurring sugars plus 4 grams of added sugar).