A cistern has three pipes A, B and C. Pipes A and B can fill it in 3 and 4 h, respectively, while pipe C can empty the completely filled cistern in 1 h. If the pipes are opened in order at 3:00 pm, 4:00 pm and 5:00 pm, respectively, at what time will the cistern be empty?

A) 6:15 pm

B) 7:12 pm

C) 8:12 pm

D) 8:35 pm

Correct answer:
B) 7:12 pm

Description for Correct answer:
Let the cistern gets emptied in m 3:00 pm. Work done by A in m h, by (m-1)hand byC in(m-2) h=0 =9 2 + 2.1 _ (m z) = o 3 4 => 4m+3(m-1)-12(m-2)=O -~ 5m = 21

m=4h12min Required time = 7 ; 12 pm.

Please provide the error details in above question

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