Which of the following options will complete the given series?
$1, 6, 15, (. . . ?), 45, 66, 91$ .

25

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26

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27

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28

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Solution

The correct option is A $28$
Solution
$1, 6, 15, ?, 45, 66, 91$
Now,
$6 - 1 = 5 = 1 \times 4 + 1$
$15 - 6 = 9 = 2 \times 4 + 1$
$66 - 45 = 21 = 5 \times 4 + 1$
$91 - 66 = 25 = 6 \times 4 + 1$
Therefore for missing number
$15 + 3 \times 4 + 1 = 28$
Therefore the missing number is $28$

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