Choose the number that will replace the question mark (?) in the following series.
3, 6, 4, 8, 6, 12, 10, 20, ?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 18
The steps are not all of one kind, so test two operations in turn: ×2 and −2 alternately.
\(3\times2=6\), \(6-2=4\)
\(4\times2=8\), \(8-2=6\)
\(6\times2=12\), \(12-2=10\)
\(10\times2=20\)
The next step is −2: \(20-2=18\)
So the missing number is 18.