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work problem-confusion

by mehrasa » Mon Sep 19, 2011 9:01 am
Jim can assemble b crates an hour and Robert can assemble 2b crates in an hour. Working together at this rate, how many crates can they assemble in four hours?

A) 3b

B) 6b

C) 8b

D) 12b

E) 18b



i solved this Q like this 1/b+1/2b = 1/X ==> 3/2b= 1/X ==> x=2b/3 in 1 hour finish 2/3b
in 4 hours ?? 2/3b*4=8/3b
could you please what's wrong with my solution? what i do not know that i can not solve some of the problems related to this topic
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by cans » Mon Sep 19, 2011 9:08 am
J-b and R-2b
thus total=3b/hour
thus in 4hours: 3b*4 = 12b
IMO D

the way u did is done for time and not for rate...
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by Abhishek009 » Mon Sep 19, 2011 9:15 am
mehrasa wrote:Jim can assemble b crates an hour and Robert can assemble 2b crates in an hour. Working together at this rate, how many crates can they assemble in four hours?

A) 3b

B) 6b

C) 8b

D) 12b

E) 18b
Consider it simply as a problem of addition and multiplication....

Efficiency of Jim is b units/hr

Efficiency of Robert can 2b units/hr

So combined efficiency / hr is 3b units


So in 4 hours they produce 3b*4 = 12b units....
Abhishek
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by mehrasa » Mon Sep 19, 2011 9:17 am
then when we can use this formula 1/a+1/b = 1/c
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by knight247 » Mon Sep 19, 2011 9:24 am
@mehrasa

Plz check out this link. It will help you with your doubt about rates. Read the 8th point

https://blog.gmatboost.com/2011/09/17/me ... ame-thing/

You may want to subscribe to that blog as well as it has lots of detailed info that will help you in quant.
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by mehrasa » Mon Sep 19, 2011 10:39 am
thnx for ur point.. i already signed up at this website but i did not receive any email so far.. how i can subscribe to be informed about the latest tips and Qs?
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by mehrasa » Mon Sep 19, 2011 11:07 am
knight247 wrote:@mehrasa

Plz check out this link. It will help you with your doubt about rates. Read the 8th point

https://blog.gmatboost.com/2011/09/17/me ... ame-thing/

You may want to subscribe to that blog as well as it has lots of detailed info that will help you in quant.
i also read the point 8 you advised me... However i did not get my answer.. I want to know when i can use simple addition when we want to find the rate of hours people work together
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