Discussion

Select the number that will replace the question mark (?) in the following series.

20, 18, 54, 51, ?, 199, 1194

Answer: D

The missing number is 204.

The series alternates subtraction and multiplication, with the step growing by one each time:

\[ \begin{aligned} 20 - 2 &= 18 \\ 18 \times 3 &= 54 \\ 54 - 3 &= 51 \\ 51 \times 4 &= 204 \\ 204 - 5 &= 199 \\ 199 \times 6 &= 1194 \end{aligned} \]

The last two terms confirm the rule. 48 would continue subtracting, 153 multiplies 51 by 3 rather than 4, and 194 fits neither step.

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