to fill a cistern, pipes p, q & r take 20, 15 & 12 minutes respectively.  the time in minutes that the three pipes together will take to fill the cistern is:----------?

10 min

5 min

20 min

15 min

to fill a cistern, pipes p, q & r take 20, 15 & 12 minutes respectively.  the time in minutes that the three pipes together will take to fill the cistern is:----------?

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