Multiple choice

The inlet to the tank can fill it in 4 hours while the outlet can empty it in 5 hrs. Both the pipes were opened at 9 AM, but after sometimes the outlet closed and it is observed that the tank was full at 5 PM. At what time was the outlet closed? एक टंकी को पाइप 4 घंटे में भर सकता है तो दूसरा पाइप उसी टंकी को 5 घंटे में खाली कर सकता है दोनों पाइप यदि सुबह 9 बजे एक साथ खोले जाये और कुछ समय बाद खाली करने वाला पाइप बंद कर दिया जाये तो टंकी 5 बजे भर जाती है तो बताइये खाली करने वाला पाइप कितने बजे बंद किया गया ?

  1. 2:20 PM

  2. 1:45 PM

  3. 2:40 PM

  4. 2:00 PM

  5. 2:15 PM
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Inlet fills at 1/4 tank/hour, outlet empties at 1/5 tank/hour. Combined net rate = 1/4 - 1/5 = 1/20 tank/hour. From 9 AM to 5 PM = 8 hours. Let outlet be open for t hours, then only inlet works for (8-t) hours. Work done: t(1/20) + (8-t)(1/4) = 1. Solving gives t = 5, so outlet closed at 9 AM + 5 hrs = 2 PM.

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