A cistern has pipes A and B that fill it in 3 hours and 4 hours, and pipe C that empties it in 1 hour. A is opened at 3 p.m., B at 4 p.m. and C at 5 p.m. When will the cistern become empty?

Question

Accepted Answer

7:12 p.m.. By 5 p.m.: A for 2 h =2/3, B for 1 h =1/4; total 11/12. After 5 p.m. net rate =1/3+1/4-1=-5/12 per hour. Time to empty =11/12÷5/12=2.2 h = 2 h 12 min → 7:12 p.m.

A cistern has pipes A and B that fill it in 3 hours and 4 hours, and pipe C that empties it in 1 hour. A is opened at 3 p.m., B at 4 p.m. and C at 5 p.m. When will the cistern become empty?

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