Pipe A can fill a tank in 45 minutes, while pipe B takes an hour to fill it…
2026
Pipe A can fill a tank in 45 minutes, while pipe B takes an hour to fill it up. Pipe A is opened first and starts filling the tank. After 18 minutes, pipe B is also opened. How many more minutes (rounded off to the next minute) are required to fill the tank completely?
Answer: D. 16 min — ConceptA pipe’s filling rate is the reciprocal of the time it takes to fill the tank. Rates of simultaneously open pipes are added. If the exact completion…
- A.
12 min
- B.
18 min
- C.
10 min
- D.
16 min
Attempted by 24 students.
Show answer & explanation
Correct answer: D
Concept
A pipe’s filling rate is the reciprocal of the time it takes to fill the tank. Rates of simultaneously open pipes are added.
If the exact completion time is fractional and the question asks for the next whole minute, use the ceiling of that time.
Application
Pipe A’s rate is 1/45 tank per minute, so in the first 18 minutes it fills 18/45 = 2/5 of the tank.
The unfilled part is 1 − 2/5 = 3/5 of the tank.
With both pipes open, the combined rate is 1/45 + 1/60 = 4/180 + 3/180 = 7/180 tank per minute.
Time for the remaining part is (3/5) ÷ (7/180) = (3/5) × (180/7) = 108/7 = 15 3/7 minutes.
Rounding 15 3/7 up to the next whole minute gives 16 minutes.
Cross-check
After 15 minutes with both pipes, the total filled is 2/5 + 15 × 7/180 = 59/60, which is short of a full tank. The tank becomes full during the 16th additional minute, confirming 16 minutes.