To answer that question, you should keep in mind that 1g of fat has 9 calories, 1g of carbohydrate has 4 calories and 1g of protein has 4 calories. Thus, we can consider that:
55 grams of carbohydrates x 4 calories = 220 calories.
15 grams of protein x 4 calories = 60 calories.
12 grams of fat x 9 calories = 108 calories.
220 + 60 + 108 = 388 calories.
With that, we can say that the total calories of the burrito is equal to 388.
With that we can calculate the percentages as follows.
% carbohydrates. (220 calories x 100%) / 388 calories = 56.7%
% of proteins. (60x100%) / 388 = 15.4%
% of fats. (108X100%) / 388 = 48%