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The key here is to notice that Set A is an evenly spaced sequence, and thus you can easily solve for its sum. The median will be equal to

first term + last term/2

3x+(3x-2)/2=3x-10

Although there is a formula to figure out how many terms there are, it is easiest in this case to count. There are six terms in the set. The sum of the terms in the set is the average value of the terms (3x − 10) times the number of terms (6). (3x − 10) × 6 = 18x − 60 Rewrite the quantities: Set A is comprised of the following terms: (3x), (3x − 4), (3x − 8), (3x − 12), (3x − 16), and (3x − 20)

QA = 18x-60
QB = 18x-70

Be careful. Quantity A is subtracting a smaller number (60) than is Quantity B (70), and so has a larger value. Therefore, Quantity A is greater.