Numerical Ability
ID: #6031 | Difficulty: medium
AI Quick Answer:
The correct option for this defence exam question is (A) which is 21.6 km/h. Detailed proof and explanation are supplied below.
A train of length 120 meters passes a telegraph post in 20 seconds. Find the speed of the train in km/h.
Key Takeaways for NDA & CDS Candidates
- Ensure you recall the standard formulas for Numerical Ability during UPSC exam preparation.
- Eliminating obviously incorrect choices based on dimensional analysis or limits saves valuable seconds.
- Expert tip: Written defence mock tests should be practiced weekly with strict negative marking rules.
Correct Answer: (A)
21.6 km/h
Step-by-Step Explanation
Speed of the train is given by Distance / Time.
Here, Distance = Length of train = 120 meters.
Time = 20 seconds.
Speed in m/s = 120 / 20 = 6 m/s.
To convert m/s to km/h, multiply by 18/5 (or 3.6):
Speed in km/h = 6 \times 3.6 = 21.6 km/h.
Here, Distance = Length of train = 120 meters.
Time = 20 seconds.
Speed in m/s = 120 / 20 = 6 m/s.
To convert m/s to km/h, multiply by 18/5 (or 3.6):
Speed in km/h = 6 \times 3.6 = 21.6 km/h.
| Question Metadata Summary | |
|---|---|
| Exam Category | UPSC NDA / CDS / AFCAT / Agniveer |
| Subject Unit | AFCAT Questions |
| Sub-topic / Module | Numerical Ability |
| Correct Option | Option (A) |
| Target Year | UPSC Defence Exams 2026 |