Walking up a moving escalator — Aptitude Question
Walking up a moving escalator: A man walking up a stationary escalator takes 90 seconds. The escalator moving with… Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Hard
- Time target: 120 seconds
Question
A man walking up a stationary escalator takes 90 seconds. The escalator moving with him standing still takes 60 seconds. How long does he take walking up the moving escalator?
Options
- 30 s
- 36 s
- 45 s
- 50 s
Show the answer and the worked solution
Correct answer: B. 36 s
Worked solution
- Take the escalator as 180 steps. Walking alone: 2 steps/s. Escalator alone: 3 steps/s.
- Together: 5 steps/s.
- Time = 180 ÷ 5 = 36 s.
Answer: 36 s.
The idea
An escalator problem is a time-and-work problem in disguise: the man and the escalator are two workers whose rates add. Take the length as the LCM of the two times.
Shortcut. Two times combine like parallel resistors: 90 × 60 ÷ (90 + 60) = 5,400 ÷ 150 = 36 s.
Common trap. 30 s — subtracting the times (90 − 60), which treats the escalator as working against the man; moving with him, the rates add.