A slower speed costs time — Aptitude Question
A slower speed costs time: A train travelling at 60 km/h reaches its destination on time. Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Hard
- Time target: 105 seconds
Question
A train travelling at 60 km/h reaches its destination on time. If it runs at 50 km/h it arrives 20 minutes late. What is the distance?
Options
- 80 km
- 90 km
- 100 km
- 120 km
Show the answer and the worked solution
Correct answer: C. 100 km
Worked solution
- d/50 − d/60 = 1/3 hour.
- Common denominator 300: (6d − 5d)/300 = 1/3 → d/300 = 1/3.
- d = 100 km.
Answer: 100 km.
The idea
Set the difference of the two times equal to the delay. Working in hours throughout avoids a unit slip at the end.
Shortcut. Distance = (product of speeds ÷ difference of speeds) × delay = (3,000 ÷ 10) × 1/3 h = 100 km.
Common trap. Writing 20 minutes as 0.2 h gives d/300 = 0.2 and d = 60 km; 20 minutes is 1/3 h, so the equation is d/50 − d/60 = d/300 = 1/3.