Three pipes A, B and C attached to a cistern. A and B can fill it in 20 min and 30 min respectively while C can empty it in 15 min. If A, B and C kept opened successively for 1 min each, how soon the cistern will be filled?

167 min

ATQ,

1st min tank fill by A = 3 ltr

2nd min tank fill by B = 2 ltr

3rd min tank empty by C = 4 ltr

So, after 3 min tank will be filled = 5 - 4 = 1 ltr.

Therefore,

Tank fill 1 ltr ⇒ in 3 min

Tank fill 55 ltr ⇒ in 165 min

Then, in next 2 min tank will be filled 5 ltrs by pipes A and B.

Therefore,

Tank will be filled completely in 165 + 2 = 167 min.