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To fill an empty cistern, we can use pipe A which would take 20 minutes, or we can use pipe B which would take 15 minutes, or we can use pipe C which would take 12 minutes.

If we turned on all three pipes, how long will it take to fill up the empty cistern?

 Dec 29, 2021
 #1
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1/ ( 1/20 + 1/15 + 1/12) = 120/24 minutes = 5 minutes

 Dec 29, 2021
 #2
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That answer is wrong.

Guest Dec 29, 2021
 #3
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Sorry.....it is correct :

    in 5 minutes   A will fill  5/20 th of the cistern    B will fill 5 / 15 th of the cistern and  C will fill 5 /12 ths of the cistern

                                         5/20                            +             5/15                                              + 5/12        = 1 complete cistern 

Guest Dec 29, 2021

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