Select the option that can replace the question mark (?) in the following series. OA33, RD36, UG39, XJ42, ?

Correct — B. Rule: first letter +3, second letter +3, number +3 at every step.

O(15) → R(18) → U(21) → X(24) → 27, and 27 − 26 = 1, so the letter is A.
A(1) → D(4) → G(7) → J(10) → M(13).
33 → 36 → 39 → 42 → 45.

Recombine the three tracks and the missing term is AM45, option (b).

The single awkward step is the first letter. X is 24, and 24 + 3 = 27 runs past Z, so you subtract 26 and wrap round to A.

Why the others are wrong

• (a)
  BO43 — BO43 moves the first letter X → B, a step of 4, and lifts the number by only 1. Every track in this series steps by 3.
• (c)
  BN44 — BN44 steps the first letter by 4 and the second by 4, J → N, while the number rises by 2. Three different step sizes, none of them the required 3.
• (d)
  CN41 — CN41 goes X → C, a step of 5, and J → N, a step of 4, and its number falls from 42 to 41. Nothing in this series decreases.

An alphanumeric series carries independent tracks — here the first letter, the second letter and the number — and each track usually moves by its own constant.

Solve the tracks separately and recombine only at the end. Mixing them is how a solvable item turns into a guess.

Converting letters to positions is what makes the arithmetic visible, and the wrap-around is where it bites: past Z you subtract 26 and continue from A.

Key facts

• O, R, U and X sit at positions 15, 18, 21 and 24, a constant step of 3.
• A, D, G and J sit at positions 1, 4, 7 and 10, the same step of 3.
• Past Z the letter count wraps, so position 27 is position 1, which is A.
• The numeric track runs 33, 36, 39, 42 and then 45.

Common traps

• Stopping at position 27 and picking a letter near Z instead of wrapping to A
• Assuming the second letter must move by the same amount as the number without checking
• Choosing an option that matches two tracks and ignoring the third

No directly related past PYQ was found.