Wrong Number Series: Finding the Odd Term

In a wrong-number series the sequence follows a clean rule except for one term that breaks it, and your job is to name that intruder. This is the most common series format in SBI Clerk and it rewards a calm, systematic check rather than a lucky glance, because the wrong term is deliberately placed to look plausible. Treat it as a two-part task: first discover the rule the series is trying to follow, then find the one term that quietly disobeys it, and never let the second part begin before the first is settled.

The Method: Establish the Rule From the Safe End

Do not test the wrong term first. Instead, look at the first three or four terms, decide the rule they imply, and then walk forward checking each term against that rule. The first term that fails to fit is your answer. If the early terms disagree among themselves, build the rule from the last few terms backward instead — one end of the series is always clean.

Shortcut: Predict Then Compare

Once you fix the rule, predict what each term should be and compare with what is printed. The mismatch jumps out immediately. This 'predict then compare' loop is faster and far more reliable than trying to feel which number looks out of place, which is exactly the instinct the examiner designs the question to fool.

Fix rule from clean terms → predict each term → the mismatch is the wrong number

Solved Examples

Example 1: 2, 6, 18, 54, 150, 486. Rule is ×3, so after 54 we expect 162, not 150 — the wrong term is 150.

Example 2: 3, 4, 8, 17, 33, 58. Gaps are +1, +4, +9, +16, +25 (squares). Check: 3, 4, 8, 17, 33, 58 — 4+4=8, 8+9=17, 17+16=33, 33+25=58, all fit, so refine an altered variant; if 34 appeared instead of 33 it would be the wrong term. Pattern discipline is the point.

Example 3: 5, 11, 23, 48, 95. Rule ×2+1: 5→11, 11→23, 23→47 expected but 48 printed, so the wrong term is 48.

✗ Guessing that 150 'looks wrong' by feel  |  ✓ Applying ×3: 54 × 3 = 162 ≠ 150, so 150 is wrong

Common Traps the Examiner Plants

The wrong term is never a wild outlier; it is chosen to sit close to the correct value so it survives a casual look. Two traps recur. First, the intruder appears early — often the second or third term — so candidates who fix their rule from the front lock onto the wrong pattern; guard against this by cross-checking your rule against the last two terms as well. Second, the series uses a mixed rule so the differences look irregular even when correct, tempting you to declare a valid term 'wrong'. The cure for both is the same: derive the rule from whichever end is internally consistent, then let the single mismatch reveal itself instead of trusting your eye.