time and work

Question

Two pipes A and B can fill tank in 10 and 20 min respectively.if both pipes are opened simultaneously ,after how many minutes should pipe B be closed so that the tank is full in 8 min?

Ans: 4 min

Answer 1

Pipe A fill tank in --->>>10 minutes

Pipe B fill tank in--->>>20 minutes

Target --->>> We have to fill the tank in 8 minutes

Given --->>> A is open for 8 minutes and B is closed after some time lets assume it 'x'

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so A fill one tank in --->>>10 minutes

so in 1 minute it will fill --->>>1/10 part of tank

so in 8 minute it will fill --->>>8/10 part of tank

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B fill one tank in --->>>20 minutes

so in 1 minute it will fill --->>> 1/20 part of tank

so in x minute it will fill--->>> x/20 part of tank

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its given that in 8 minutes the tank is full so ultimately

x/20 + 8/10 =1

so x=4 minutes