Aptitude Test: Sequence and Coding Under Constraint
A ground-duty technical candidate faces a reasoning set. Series: 3, 7, 16, 35, 74, ?. Coding: if FLIGHT is coded as GMKLIW, how is SORTIE coded? Analogy: Radar : Detection :: Sonar : ?. The candidate has 90 seconds for all three and notices the series looks 'almost double plus one' but not quite.
1. Solve the number series and state the rule, showing why the 'double plus one' guess fails.
Read the worked answer
Test 'double plus one':
- 3×2+1 = 7 ✓
- 7×2+1 = 15 ✗ (given 16)
So the rule is not constant. Test 'double plus increasing increment':
- 3×2+1 = 7 (increment +1)
- 7×2+2 = 16 (increment +2)
- 16×2+3 = 35 ✓
- 35×2+4 = 74 ✓
- 74×2+5 = 153
Rule: multiply by 2 and add 1, 2, 3, 4, 5 in turn. Answer = 153.
Why the naive guess fails: the first step happens to fit 'double plus one', which is exactly the trap. A single confirming term is not a rule - you need at least two consecutive confirmations before committing. This is the reasoning equivalent of not calling a track hostile on one sweep of data.
2. Solve the coding question and the analogy, and explain the general method for each.
Read the worked answer
Coding - FLIGHT → GMKLIW: Compare letter by letter:
- F→G (+1), L→M (+1), I→K (+2), G→L (+5), H→I (+1), T→W (+3)
That is irregular, so test a simpler hypothesis: each letter shifted +1, +1, +2, +5, +1, +3 is unlikely to be the intended rule. Re-examine: F(+1)=G, L(+1)=M, I(+2)=K, G(+5)=L, H(+1)=I, T(+3)=W. The shifts 1,1,2,5,1,3 do not form a clean pattern.
Safer hypothesis: the code may be a positional cipher (e.g. reverse alphabet or keyboard shift). Without a second example, the rule is underdetermined - multiple rules fit one pair. The correct exam behaviour is to flag it and, if forced, choose the most common convention (uniform +1), giving SORTIE → TPSUJF.
Analogy - Radar : Detection :: Sonar : ? Relationship: instrument → what it is used to detect/measure. Sonar detects sound underwater / submarines. Answer: submarine detection (or 'underwater ranging').
Method: for coding, demand at least two examples before trusting a rule; for analogies, name the relationship in a sentence before choosing.