One pipe finishes the job — Time & Work Aptitude Question
One pipe finishes the job: Two pipes can fill a tank in 10 minutes and 15 minutes. Both are opened for 4 minutes,… Answer and worked solution on CodeKairo.
- Topic: Time & Work
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 90 seconds
Question
Two pipes can fill a tank in 10 minutes and 15 minutes. Both are opened for 4 minutes, then the first is closed. How much longer does the second take to fill the tank?
Options
- 4 min
- 5 min
- 6 min
- 7 min
Show the answer and the worked solution
Correct answer: B. 5 min
Worked solution
- In 4 minutes: 4/10 + 4/15 = 6/15 + 4/15 = 10/15 = 2/3 filled.
- Remaining = 1/3, at the second pipe's rate of 1/15 per minute.
- Time = (1/3) × 15 = 5 minutes.
Answer: 5 min.
The idea
Compute the fraction filled together, subtract from one, and divide by the surviving rate. The answer asked for is the extra time, not the total.
Shortcut. The second pipe runs throughout and must supply 1 − 4/10 = 3/5 of the tank, which is 9 of its 15 minutes; 9 − 4 = 5 more.
Common trap. 4 min — tempting because it echoes the 4 minutes already run; the joint run filled 2/3, and the last third at 1/15 a minute takes 5.