In one day, Joe consumes 530 calories by drinking 1 serving of juice, 2 servings of milk, and 1 soda. Darius consumes 370 calories by drinking 2 servings of juice and 1 serving of milk. Marian consumes 510 calories by drinking 3 servings of milk and 1 soda. Using matrices to solve, how many calories are in 1 serving of milk?

Let j, m, and s represent the calories in one serving of juice, milk, and soda. The equations are j + 2m + s = 530, 2j + m = 370, and 3m + s = 510. From the second and third equations, j = (370 − m)/2 and s = 510 − 3m; substituting these into the first gives 695 − 1.5m = 530, so m = 110. Therefore, one serving of milk contains 110 calories, choice B.

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