Solved UPSC CDS/OTA Question
A solved UPSC CDS/OTA PYQ — decoded step-by-step.
Solve it the standard way once — then learn the shortcut.
Q · Time and WorkMedium · 45s
Two pipes can fill a tank in 20 hours and 25 hours. They are opened alternately for an hour each starting with the first pipe. How long will it take to fill the tank?
TopicTime and Work
Question Typemultiple choice
DifficultyMedium
Marking+1 / −0.25
Test Duration30 min
Step-by-Step Solution
The complete worked solution.
Every step explained — no skipped algebra.
In 2 hours: 1/20+1/25=9/100. 100/9 ≈ 11.11 cycles needed → ~46 hours.
Final Answer: Option C — 46 hours