It takes the average dryer 80 minutes to dry one full load : GMAT Problem Solving (PS)
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# It takes the average dryer 80 minutes to dry one full load

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It takes the average dryer 80 minutes to dry one full load [#permalink]

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24 Jul 2010, 07:38
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Question Stats:

36% (04:18) correct 64% (02:51) wrong based on 25 sessions

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It takes the average dryer 80 minutes to dry one full load of Bob's laundry. Bob has one average dryer, and also decides to try one "Super Jumbo-Iron Dryer-Matic", which is faster than the average dryer by a ratio of 5 : 4. Bob has three loads of laundry, and both machines are so precise that he can set them to the minute. How many minutes does it take for him to dry his three loads?

Assume no time passes between loads and he can use both machines concurrently. also assume he can run partial loads and can switch from one machine to the other.

Answer is 107 minutes. without switching loads and partial loads not allowed, then it's 128. I understand the 128, but I'm too dense to get the 107. Please, please explain...thanks!
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Re: super hard - 107 minutes word problem [#permalink]

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24 Jul 2010, 08:17
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Work completed (Speed) by Average Drier = 1/80 in a min
Ratio between Speed (avg): Speed (super) = 5:4
Work completed (speed) of super = 5/4 * (1/80) = 1/64
Work completed by both super and avg in a min = 1/80 + 1/64 = 144/ (80*64)
Time taken to complete 1 load = (80*64)/144
Time taken to complete 3 loads = ((80*64)/ 144 ) * 3 = 106.66 ~ 107 mins
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Re: super hard - 107 minutes word problem [#permalink]

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26 Jul 2010, 01:15
this is not hard, but im confused with the unit ''load''
do we have to put the one whole load in the dryer each time or we can still seperate them into pieces of clothes?
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Re: super hard - 107 minutes word problem [#permalink]

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17 Jul 2011, 00:09
extended solution to the case modification of ratio problem #14, MGMAT Strategy World Problems 4th Edition:
the average method is pretty easy to understand, but it is possible to stick to the grid method as well:

R * T = W
1/64 * x = 5/9 --> (5/9 * 64/1) * 3 = 320/3 (106 2/3)
1/80 * x = 4/9 --> (4/9 * 80/1) * 3 = 320/3 (106 2/3)

Total of work must be 1.
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Re: super hard - 107 minutes word problem [#permalink]

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17 Jul 2011, 12:51
Average dryer speed = 1/80 loads/min

Super dryer speed = Average dryer speed * (5/4) = 5/(4*80)

Total time to do 3 loads when both dryers are running = t

=> 1/80 + 5(4*80) = 3/t

=> t = 107 minutes
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Re: super hard - 107 minutes word problem [#permalink]

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17 Jul 2011, 15:00
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Time ratio Average:Super = 5:4
Obviously the average dryer will take more time than Super Dryer
So 5x=80 => x=16 mns
Now for Super dryer since it takes less time , so time for one load is 4x = 4.16=64 mns
So the qn. is what will be the time for 2 machines - 80 mns/load and 64 mns/load to handle 3 loads
(80.64/(80+64))3=80.64.3/144 = 106.67 mns ~ 107 mns

Hope this makes sense...
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Re: super hard - 107 minutes word problem [#permalink]

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17 Jul 2011, 20:21
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Expert's post
knabi wrote:
it takes the average dryer 80 mins to dry one full load of bob's laundry. bob has one average dryer, and also decides to try one "super" dryer, which is faster than the average dryer by a ratio of 5:4. bob has three loads of laundry, and both machines are so precise that he can set them to the minute. how many minutes does it take for him to dry his three loads?

assume no time passes between loads and he can use both machines concurrently. also assume he can run partial loads and can switch from one machine to the other.

Answer is 107 minutes. without switching loads and partial loads not allowed, then it's 128. I understand the 128, but I'm too dense to get the 107. Please, please explain...thanks!

This is a straight forward Work-Rate problem. But the context of dryer and loads is inappropriate.
Say a machine makes 100 nails in 100 minutes. When we assume serial processing, we can say that it makes 1 nail in 1 minute. But we know that this is not true for dryers. If 80 clothes take 80 minutes to dry, 1 shirt will not dry up in a minute. The dryer does parallel processing. This is not a good question.
When the question says 'assume he can run partial loads', it still doesn't mean that the partial loads take less time. We know they want us to assume that half a load will take half the time because otherwise the information that he can run partial loads is useless.

Basically, the question is the following:
A regular machine completes a job in 80 minutes. A super fast machine takes only 64 minutes to complete it. How long will the two of them take together if they need to complete 3 jobs?
This can be easily done as shown in the posts above.
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Manager Joined: 14 Apr 2011 Posts: 199 Followers: 2 Kudos [?]: 23 [0], given: 19 Re: super hard - 107 minutes word problem [#permalink] ### Show Tags 18 Jul 2011, 03:27 Thanks Karishma for pointing out the catch and also for explaining serial vs. parallel processing concept in rate problems! I did not think about it. _________________ Looking for Kudos Senior Manager Status: mba here i come! Joined: 07 Aug 2011 Posts: 270 Followers: 43 Kudos [?]: 1058 [0], given: 48 Re: super hard - 107 minutes word problem [#permalink] ### Show Tags 27 Aug 2011, 00:24 for 1 load, slower machine takes = 80mins faster machine takes = 80*4/5 = 64mins together for 1 load = 80*64/(80+64) for 3 loads = 80*64*3/144 = 320/3 = 106.6667 OR 107 complete mins _________________ press +1 Kudos to appreciate posts Intern Joined: 20 Oct 2011 Posts: 5 Followers: 0 Kudos [?]: 0 [0], given: 3 Re: super hard - 107 minutes word problem [#permalink] ### Show Tags 14 Nov 2011, 07:27 Hi everyone, Well I got the 107 minutes solution using all the techniques and approaches that have been shown in previous posts but... the solution from the book is 128 minutes because partial loads are not allowed and therefore the super machine would have to receive two full loads in order to get the best time. now, I found that to be "tricky" ... I mean, is this a typical Gmat question? or is it just a rare example for training with ratios? ... Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7127 Location: Pune, India Followers: 2139 Kudos [?]: 13699 [0], given: 222 Re: super hard - 107 minutes word problem [#permalink] ### Show Tags 14 Nov 2011, 20:47 danielphonics wrote: Hi everyone, Well I got the 107 minutes solution using all the techniques and approaches that have been shown in previous posts but... the solution from the book is 128 minutes because partial loads are not allowed and therefore the super machine would have to receive two full loads in order to get the best time. now, I found that to be "tricky" ... I mean, is this a typical Gmat question? or is it just a rare example for training with ratios? ... GMAT questions are not ambiguous/non intuitive. They make complete sense and different people come up with the same inferences from the given data. They will never try to trick you this way. You will know exactly what is known and exactly what you have to find out. The trick will be in 'how to get there'. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: super hard - 107 minutes word problem [#permalink]

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04 Jan 2012, 23:37
Tricky question. Thanks Karishma for explaining the concept of serial and parallel processing. 107 is the answer.
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Re: super hard - 107 minutes word problem [#permalink]

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05 Jan 2012, 00:04
107 it is...tricky
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Re: super hard - 107 minutes word problem [#permalink]

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12 Jul 2012, 21:41
VeritasPrepKarishma wrote:
danielphonics wrote:
Hi everyone,

Well I got the 107 minutes solution using all the techniques and approaches that have been shown in previous posts but... the solution from the book is 128 minutes because partial loads are not allowed and therefore the super machine would have to receive two full loads in order to get the best time.

now, I found that to be "tricky" ... I mean, is this a typical Gmat question? or is it just a rare example for training with ratios? ...

GMAT questions are not ambiguous/non intuitive. They make complete sense and different people come up with the same inferences from the given data. They will never try to trick you this way. You will know exactly what is known and exactly what you have to find out. The trick will be in 'how to get there'.

I can get 107... But what i would like to know is how 128 is arrived at??
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Re: super hard - 107 minutes word problem [#permalink]

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12 Jul 2012, 23:23
MacFauz wrote:
I can get 107... But what i would like to know is how 128 is arrived at??

IF you dont switch and partial loads are not allowed, you will put one load in normal dryer and another in super fast. Once the super fast is done, you will put another load in it.

Since time taken by normal dryer is 80 mins, time taken by super fast dryer is 64 mins (ratio of 5:4 of speed so ratio of time taken by super fast:time taken by normal = 4:5)

So time taken for 2 loads of super fast dryer = 64 + 64 = 128 mins
(in this time, the normal dryer would have finished one load and would be sitting idle for 48 mins)
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Re: It takes the average dryer 80 minutes to dry one full load [#permalink]

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30 Jul 2012, 14:37
I'm on the exact opposite side of the spectrum. I can get 128, but no idea where all the 107's are coming from.
Re: It takes the average dryer 80 minutes to dry one full load   [#permalink] 30 Jul 2012, 14:37
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