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Insert the missing number 2, 6, 12, 20, 30, 42, 56, (....)

  • A.61
  • B.64
  • C.72
  • D.70

Answer: C

Each term is a product of two consecutive integers.

\[ 1 \times 2 = 2,\quad 2 \times 3 = 6,\quad 3 \times 4 = 12,\quad 4 \times 5 = 20,\quad 5 \times 6 = 30 \]

\[ 6 \times 7 = 42,\quad 7 \times 8 = 56,\quad 8 \times 9 = \mathbf{72} \]

Equivalently the differences are the even numbers 4, 6, 8, 10, 12, 14, so the next gap is 16 and \( 56 + 16 = 72 \).

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