Time-sequence / swapping positions: In a row of children, Bhusan is 7th from the left and Motilal is 4th from the right. After they exchange seats, Bhusan becomes 15th from the left. What is Motilal’s position from the right after the swap?

Introduction / Context:
Swapping seats preserves each seat’s absolute left/right indices. Use the seat indices to deduce total N and then convert.

Given Data / Assumptions:

• Before swap: Bhusan = 7th from left; Motilal = 4th from right.
• After swap: Bhusan = 15th from left ⇒ he now occupies Motilal’s old seat.

Concept / Approach:
If Motilal’s original seat is 15th from left and also 4th from right, then N - 15 + 1 = 4 ⇒ N = 18. Motilal after swap moves to the 7th-from-left seat (Bhusan’s old seat), so his new right-rank is N - 7 + 1.

Step-by-Step Solution:

Verification / Alternative check:
Check Bhusan’s after-swap index equals 15 from left (matches given). Consistent.

Why Other Options Are Wrong:
Other values contradict the conversion formula or the given post-swap rank.

Common Pitfalls:
Mixing up “person’s rank” and “seat index,” and forgetting the +1 in conversions.

Final Answer:
Twelfth