SOLUTION: I saw this problem in my chemistry textbook: How many Calories are in 1 breakfast roll if it has 30. g of carbohydrate, 15 g of fat, and 5 g of protein? Here is my work: Carbohy

Question 333594: I saw this problem in my chemistry textbook:
How many Calories are in 1 breakfast roll if it has 30. g of carbohydrate, 15 g of fat, and 5 g of protein?
Here is my work:
Carbohydrate: 30. g (2 Significant Figures) * 4 kcal/1g = 120 kcal (2 Sig Figs)
Fat: 15 g (2 Sig Figs) * 9 kcal/1g = 135 kcal (3 Sig Figs)
Protein: 5 g (1 Sig Fig) * 4 kcal/1g = 20 kcal (1 Sig Fig)
Total caloric content: 275 kcal (Calories)
The Book's answer is:
280 kcal.
My question is: How do you get the right number of sig figs; when you have addition and multiplication? The question could also go for addition or subtraction combined with either multiplication or division.
Answer by jrfrunner(365) (Show Source): You can put this solution on YOUR website!
in your information, such as grams of carbohydrates you show 30.g, so you are not showing the fraction of grams, ditto for fat and protein, so that is probably where your computations did not come out to the book's answer of 280.
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by the way, the relationship of calories to grams of carbo, fat, protein is "calories" not Kcal which implies 1000 calories per gram.
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as far significant figures, you should always carry out yoru computations to at least one more decimal place than the original data, and then you can round "at the end" of the computation. Otherwise you carry rounding errors which can magnify with each calculation.
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