PREVIOUSLY ASKED IN:
PSC Miscellaneous Prelims 2018
Answer
15 sec
Explanation
1 minute equals 60 seconds. The cistern fills 4 out of 5 parts in 60 seconds. Therefore, the time taken to fill 1 part is 60 / 4 = 15 seconds. The remaining part of the cistern is (1 - 4/5) = 1/5th, which is exactly 1 part. Thus, it will take an additional 15 seconds to fill the rest of the cistern.
Key Points
- > Time taken for 4/5 of the tank = 60 seconds.
- > Time taken for 1 full tank = (60 × 5) / 4 = 75 seconds.
- > Remaining portion to be filled = 1 - 4/5 = 1/5.
- > Time for remaining portion = 75 × (1/5) = 15 seconds.
- > The unitary method is the fastest way to solve basic pipe and cistern problems.
Additional Information
Fraction and Time Equivalence
| Fraction | Percentage | Time (out of 60s) |
|---|---|---|
| 1/5 | 20% | 12 seconds |
| 2/5 | 40% | 24 seconds |
| 3/5 | 60% | 36 seconds |
| 4/5 | 80% | 48 seconds (if total is 60s) |
Memory Tips
- Numerator Trick: 4 parts = 60s. So, 1 part = 15s. The remaining is 1/5 (which is 1 part). Answer is 15s immediately.