Question

How long will two pipes together take to fill a cistern which they can separetly fill in 20 minutes and 25 minutes ?​

Answer 1

Answer:

Efficiency of pipe A = 1/20 th part of cistern per minute

Efficiency of pipe B = 1/25 th part of cistern per minute

Combined efficiency of A and B = 1/20 + 1/25 = 9/100 th part of the cistern per minute

Time of filling the cistern = 100/9

Tap B is closed 5 minutes before the filling that is the two taps run together for (100/9 - 5) ie 55/9 minutes

= Part of cistern filled in 55/9 minutes = 9/100*55/9 = 11/20 th part

Part of the cistern remained to be filled = 1 - 11/20 = 9/20 th part

The time taken by A = 9/20*20 = 9 minutesStep-by-step explanation:

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Answer 2

Answer:

$\huge\underline\bold {Answer:}$

Part of cistern filled by 1st pipe in 1 min = 1/20

Part of cistern filled by 2nd pipe in 1 min = 1/25

Therefore part of cistern filled by both the pipes together in 1 min = 1/20 + 1/25

= 9/100

Hence the cistern will be filled by both the pipes together in 100/9 min.