Solved UPSC CDS/OTA Question
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Q · Time and WorkMedium · 45s
A cistern has two inlet pipes. One fills it in 9 hours, the other in 12 hours. An outlet can empty the full cistern in 18 hours. If all three are opened together, how long will it take to fill the cistern?
TopicTime and Work
Question Typemultiple choice
DifficultyMedium
Marking+1 / −0.25
Test Duration30 min
← Previous questionQ 60 / 60
Step-by-Step Solution
The complete worked solution.
Every step explained — no skipped algebra.
Net rate = 1/9 + 1/12 - 1/18 = (4+3-2)/36 = 5/36 → Time = 36/5 = 7.2 hours.
Final Answer: Option B — 7.2 hours