5) how hard do these machines work?

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ern5231: Master | Next Rank: 500 Posts; Posts: 226; Joined: Sun Aug 09, 2009 4:34 am

by ern5231 » Fri Apr 30, 2010 10:34 pm

Two machines A and B drain water from a tank. A worked alone for 4 hours to
Drain half of the water in the tank. Then, B joined and the remaining water could be drained in 3 hours. In how many hours could B drain the water alone?

I picked up this question from net. Don't have the options. I worked out the answer 24. Wts your opinion?

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Source: Beat The GMAT — Problem Solving | Fri Apr 30, 2010 10:34 pm

beat_gmat_09: Master | Next Rank: 500 Posts; Posts: 437; Joined: Sat Nov 22, 2008 5:06 amFollowed by: 1 member; Location: India; Thanked: 50 times; GMAT Score:580

by beat_gmat_09 » Fri Apr 30, 2010 11:00 pm

ern5231 wrote:Two machines A and B drain water from a tank. A worked alone for 4 hours to
Drain half of the water in the tank. Then, B joined and the remaining water could be drained in 3 hours. In how many hours could B drain the water alone?

I picked up this question from net. Don't have the options. I worked out the answer 24. Wts your opinion?

1/4+1/x=1/3
x=12 B can drain half water in 12 hrs therefore alone it can drain in 24 hrs.

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harshavardhanc: Legendary Member; Posts: 526; Joined: Sat Feb 21, 2009 11:47 pm; Location: India; Thanked: 68 times; GMAT Score:680

by harshavardhanc » Sat May 01, 2010 3:31 am

ern5231 wrote:Two machines A and B drain water from a tank. A worked alone for 4 hours to
Drain half of the water in the tank. Then, B joined and the remaining water could be drained in 3 hours. In how many hours could B drain the water alone?

I picked up this question from net. Don't have the options. I worked out the answer 24. Wts your opinion?

A can drain half of the quantity in 4 hrs i.e 25% in one hour.

together with B, he's able to drain half the water in 3 hrs, i.e efficiency is increased to 33.33% or by 8.33%. which is 1/12.

hence, B can drain half the tank in 12 hours or full capacity of the tank in 24 hours.

Regards,
Harsha

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this_time_i_will: Master | Next Rank: 500 Posts; Posts: 268; Joined: Wed Mar 17, 2010 2:32 am; Thanked: 17 times

by this_time_i_will » Sat May 01, 2010 6:02 am

ern5231 wrote:Two machines A and B drain water from a tank. A worked alone for 4 hours to
Drain half of the water in the tank. Then, B joined and the remaining water could be drained in 3 hours. In how many hours could B drain the water alone?

I picked up this question from net. Don't have the options. I worked out the answer 24. Wts your opinion?

A simple formula that can be handy for any similar type of problem is:

(A+B) togethar = AB/(A+B) .
6 = 8B/(8+B)
B = 24.

My Blog:
https://www.iwantutopia.blocked/

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