Question

three pipes A, B and C is attached too cistern . A can fill ut in 20 minutes and B can fill it in 30 minutes, C is a waste pipe, After opening both the pipe A and B , Riya leaves the cistern to fill and return when the cistern is supposed to be filled . But she found that waste pipe C had been left open , she close it and now the cistern takes 5 minutes more to fill. in how much time the pipe C can empty the full cistern?
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Answer 1

Answer:

Let pipe C alone can empty the cistern in x min.

A fills cistern in 1 min = 110

B fills cistern in 1 min = 115

A and B together fill in 1 min =10×1510+15=15025=6min

Since, waste pipe was left open for 6 min then,

6 min, 6x

part of cistern will be emptied by C

Now,

6x

part of the cistern would be filled by A and B in 2 min.

Hence,

cistern will be filled in 3x

min. And

x3

= 6

x = 18 min.