Which number will come next in the following series 3, 8, 5, 7, 8, 5, 12

The series is two interleaved sequences, so split it by position. Terms at odd positions are 3, 5, 8, 12, rising by 2, 3, 4. Terms at even positions are 8, 7, 5, falling by 1, then 2. The next term is at position 8, which is even, so continue the even sequence by subtracting 3: 5 - 3 = 2.

Why the other options fail:

• B) 3: 3 is only the first term. Nothing in either sequence returns to it.
• C) 4: no consistent difference in either sequence produces 4.
• D) 1: this would need a drop of 4 after 5, which breaks the -1, -2, -3 pattern.

Core concept: alternating or interleaved series. When a single rule is not visible, separate the terms by position and test each subsequence for its own difference pattern. This is the same discipline needed for data interpretation questions in Paper I and for reasoning in CSAT.

Trap: Trying to find one rule across all seven terms, then guessing. Many aspirants also take the odd-position rule (17) by mistake, because it is the more obvious continuation of the rising numbers.