Two large pipes X & Y can separately fill a cistern in 20 minutes and 30…
2024
Two large pipes X & Y can separately fill a cistern in 20 minutes and 30 minutes. Together, they can fill the cistern in:
- A.
1 hour 31 minutes
- B.
12 minutes
- C.
2 hours 12 minutes
- D.
2 hours
Attempted by 56 students.
Show answer & explanation
Correct answer: B
Key idea: use work rates (fraction of the cistern filled per minute).
Pipe X fills 1/20 of the cistern per minute.
Pipe Y fills 1/30 of the cistern per minute.
Combined rate = 1/20 + 1/30 = 1/12 of the cistern per minute.
Time to fill = 1 ÷ (combined rate) = 1 ÷ (1/12) = 12 minutes.
Check: the combined time (12 minutes) is less than the faster individual time (20 minutes), which makes sense because two pipes working together fill faster.