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Answer: (B) 10
Explanation
The relationship between the numbers follows the formula x = 2y + k, where x is the first number, y is the second number, and k is an adjustment constant. Arranging the pairs in ascending order of the first number x:
Pair 1 (11:6): 11 = 2(6) - 1, so k = -1 Pair 2 (16:8): 16 = 2(8) + 0, so k = 0
The values of x (11, 16, 21) form an arithmetic progression with a common difference of 5. Correspondingly, the values of k follow an arithmetic progression (-1, 0, 1) with a common difference of 1. Applying this pattern to the third pair (21:?):
21 = 2y + 1 20 = 2y y = 10