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a PS from Gmat

by diebeatsthegmat » Wed Aug 31, 2011 6:06 pm
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?

the answer is 107 and my answer is 128... from which it says " how long does it takes both dryers fast and average run TOGETHER to do the job well done" ? the question is just said " how many minutes does it take for him to dry his three loads" i dont understand... can you please explain?
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by mukgera » Wed Aug 31, 2011 7:23 pm
The questions simply talks about what is the total time taken when we have 3 loads and two dryers are running together.

Now avg dryer takes 80 min to complete one load of work
Ratio of speed for Super dryer : avg dryer = 5:4

This problem can be solved by taking some number for the clothes in the load.Which could be taken with the LCM of speed = 20 * minutes taken by first machine (took this number because every calculation will give integer then) = 20 * 80 = 1600 clothes / load

So Avg dryer speed = 1600 /80 = 20 clothes/min
Speed of Super dryer would be = 20 * 5/4 = 25 clothes /min

So together they can handle = 20 +25 = 45 clothes/min

So If we have 3 loads = 3* 1600 = 4800 clothes

So time taken = 4800 / 45 = 106.66 min ~ 107 (the machine can be set to minutes granularity only)
Hope this will clear the doubt.
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by SticklorForDetails » Wed Aug 31, 2011 9:23 pm
diebeatsthegmat 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?

the answer is 107 and my answer is 128... from which it says " how long does it takes both dryers fast and average run TOGETHER to do the job well done" ? the question is just said " how many minutes does it take for him to dry his three loads" i dont understand... can you please explain?
I agree that this question is poorly-worded; the solution used by mukgera above, which is absolutely correct, assumes that the question means, "How many minutes does it take him to dry his three loads using both machines together for the entire time?" The problem is this is illogical -- how can one divide three loads evenly between two dryers? They don't work "together" the way most machines do. Thus it's a flawed question, and you won't see one like it on the GMAT. On the test, the question would read as I wrote it here, and the answer would be Mukgera's absolutely.

As always, please post the source of the question!
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by diebeatsthegmat » Thu Sep 01, 2011 11:39 am
SticklorForDetails wrote:
diebeatsthegmat 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?

the answer is 107 and my answer is 128... from which it says " how long does it takes both dryers fast and average run TOGETHER to do the job well done" ? the question is just said " how many minutes does it take for him to dry his three loads" i dont understand... can you please explain?
I agree that this question is poorly-worded; the solution used by mukgera above, which is absolutely correct, assumes that the question means, "How many minutes does it take him to dry his three loads using both machines together for the entire time?" The problem is this is illogical -- how can one divide three loads evenly between two dryers? They don't work "together" the way most machines do. Thus it's a flawed question, and you won't see one like it on the GMAT. On the test, the question would read as I wrote it here, and the answer would be Mukgera's absolutely.

As always, please post the source of the question!
ehhh i am out of sources to study thus i am starting to do each math discussed in the forum thus sometimes i dont know what source this question is from as usual, but knew its very odd and bad written :( thank you a lot :)
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