13 is related to 21 following certain logic. Following the same logic, 35 is related to 43. To which of the following numbers is 66 related, following the same logic? (NOTE : Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. E.g. 13 – Operations on 13 such as adding /deleting /multiplying etc., to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed.)

Correct — C. Test one operation against both given pairs before you take it anywhere near 66.

21 − 13 = 8
43 − 35 = 8

Rule: add 8 to the whole number. The stem's NOTE bars breaking a number into digits, so 13 is worked as thirteen, not as 1 and 3.

66 + 8 = 74 → option (c).

Why the others are wrong

• (a)
  71 — 71 is 66 + 5, and a step of 5 fails on the first pair: 13 + 5 = 18, not 21.
• (b)
  73 — 73 is 66 + 7, and a step of 7 fails on the first pair: 13 + 7 = 20, not 21.
• (d)
  72 — 72 is 66 + 6, and a step of 6 fails on the first pair: 13 + 6 = 19, not 21.

A number analogy gives you two worked pairs and asks for a third. The discipline is to propose a rule and then test it against both pairs, because a rule fitted to one pair alone can fail on the other.

Here the difference is 8 in both, which is as clean as this stem gets.

The bracketed NOTE is doing real work: it removes every digit-level rule from consideration before you start, which is why the search collapses to a single arithmetic step.

Key facts

• 13 → 21 and 35 → 43 both move by exactly +8.
• The NOTE forbids digit-level work, so reading 13 as 1 and 3 is out of bounds.
• No constant ratio fits, since 21/13 and 43/35 are not equal.

Common traps

• Subtracting in the wrong direction, taking 13 − 21 and then applying −8 to 66
• Ignoring the NOTE and adding digits, which the stem explicitly disallows
• Accepting a step that fits 35 → 43 without checking it against 13 → 21

No directly related past PYQ was found.