Discussion

Insert the missing number 10, 5, 13, 10, 16, 20, 19, (....)

Answer: B

Two series are interleaved.

\[ \text{positions 1, 3, 5, 7: } 10,\; 13,\; 16,\; 19 \quad (+3) \]

\[ \text{positions 2, 4, 6, 8: } 5,\; 10,\; 20,\; \mathbf{40} \quad (\times 2) \]

The missing eighth term belongs to the doubling series:

\[ 20 \times 2 = 40 \]

Reading the list as a single sequence gets nowhere; the alternation is the whole structure.

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