Two trains move in the same direction at 15 km/h and 21 km/h. What is their relative speed with respect to each other (in km/h)?

Introduction / Context:
Relative speed in the same direction is the absolute difference of the individual speeds. It reflects how fast the faster one gains on the slower one.

Given Data / Assumptions:

• v_fast = 21 km/h.
• v_slow = 15 km/h.

Concept / Approach:
Same direction ⇒ v_rel = v_fast − v_slow.

Step-by-Step Solution:

Verification / Alternative check:
In one hour, the faster train pulls ahead by 6 km—definitionally correct.

Why Other Options Are Wrong:
26 and 24 are sums rather than differences; 15 is just one speed; 36 is unrelated.

Common Pitfalls:
Adding speeds for same-direction motion; mixing km/h with m/s.

Final Answer:
6 km/h