og ps 234

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og ps 234

by parisfr67 » Sun Feb 24, 2008 4:15 pm
Official Guide Problem Solving Question 234:

Question:
In a certain game, a large container is filled with red, yellow, green and blue beads worth, respectively 7,5,3, and 2 points each. A number of beads are then removed from the container. If the product of the point values of the removed beads is 147,000, how many red beads were removed?

OG Answer:
From this, the red beads represent factors of 7 in the total point value of 147,000. Since 147,000 = 147(1000), and 1000 = 10 cubed, then 147 is all that needs to be factored to determine the factors of 7. Factoring 147 yields 147 = (3)(49) = (3)(7 squared). This means there are 2 factors of 7, or 2 red beads.

My Question:
The container only contains Red(7), Yellow(5), Green(3) and Blue(2) beads with their respective point value. (I am not saying there are only 7 red beads, 5 yellow beads, etc)
The total point value is 147 thousand --> 147,000
A factor of one thousand is being put aside and is not accounted for because...??....
Could someone explain to me why we are setting aside a factor of 1000 points which corresponds to a quantity of beads and settling for only 2 red beads (7 squared -- 7 being the point value for red beads). The 1000 points corresponds to some quantity of Red, Yellow, Green and Blue beads, since none of the points equate to a value of 10 (from 10 cubed), this point value must be a 'composite' value of some combination of the beads.

What am I not seeing in this problem?

Thank you in advance for your time spent on this problem

Alain


Are the 1000 beads (factored out) a factor of the beads with point value of 5 and 2? -- ie: Green(5) and Blue(2)
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by Stuart@KaplanGMAT » Sun Feb 24, 2008 7:41 pm
We know that the product of all the beads is 147000.

We also know that the only factor we really care about is 7, so we want to see how many 7s "go into" 147000.

When breaking a number down into primes, we often use a prime factor tree.

The easiest way to start to break down 147000 is 147 * 1000.

Now, we know that 7 doesn't go into 1000, so we can ignore that part of the tree. We only have to worry about the 147 branch.

We can then break down 147 as 3 * 49 and finally break down 49 as 7*7.

So, we have:

147000 = 1000 * 3 * 7 * 7

which means exactly 2 red beads.
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elementary question

by simba12123 » Tue Oct 28, 2008 6:28 am
Still cloudy on why the 1000 is a factor and being ignored. I made a mistake of reading the question too fast and not seeing that the question is asking for how many reds..

One final question is about the language the OG uses. Instead of writing 147,000. the book uses 147,000,. What is the point of this frivolous extra comma?

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Re: elementary question

by logitech » Tue Oct 28, 2008 8:05 am
simba12123 wrote:Still cloudy on why the 1000 is a factor and being ignored. I made a mistake of reading the question too fast and not seeing that the question is asking for how many reds..

One final question is about the language the OG uses. Instead of writing 147,000. the book uses 147,000,. What is the point of this frivolous extra comma?
So they can piss you off and test whether you can stay calm :lol:
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Re: elementary question

by Stuart@KaplanGMAT » Tue Oct 28, 2008 10:22 am
simba12123 wrote:Still cloudy on why the 1000 is a factor and being ignored. I made a mistake of reading the question too fast and not seeing that the question is asking for how many reds..

One final question is about the language the OG uses. Instead of writing 147,000. the book uses 147,000,. What is the point of this frivolous extra comma?
We ignore the "1000" because we're trying to determine how many times "7" goes into the number and we know that 7 does NOT go into 1000.

Basically, we're making a prime factoring tree. When you make such a tree, you can divide the original number any way you want and you always end up with the same primes at the bottom.

So, for 147000, we can factor as:

147000 = 147 * 1000 = (3*49) * (10*100) = (3) * (7*7) * (2*5) * (10*10)
= (3) * (7*7) * (2*5) * (2*5) * (2*5)

So, the prime factors of 147000 are 2, 2, 2, 3, 5, 5, 5, 7, 7

However, if we do the whole tree, we waste a ton of our time, which is why I ignored the "1000".
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Re: elementary question

by Ian Stewart » Wed Oct 29, 2008 9:14 am
simba12123 wrote: One final question is about the language the OG uses. Instead of writing 147,000. the book uses 147,000,. What is the point of this frivolous extra comma?
The comma isn't frivolous. The first comma, after '147', is part of the number, while the second comma is part of the sentence, and is not part of the notation of the number. The question says:

"If the product of the point values of the removed beads is 147,000, how many red beads were removed? "

Grammatically, there needs to be a comma after '147,000', in the same way that you would need a comma if the sentence said:

"If the product of the point values of the removed beads is greater than five, how many red beads were removed? "
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Why does it have to be 2?

by sssssss » Fri Apr 10, 2009 7:43 pm
Maybe I am missing something. I see that the answer given in the book reveals one possible answer. It could be that 2 red beads were removed. But I don't see why it has to be that EXACTLY 2 red beans were removed.

As I understand the question, we are saying that 147000 = 7R5Y3G2B, where R is the number of red removed, yellow is the number of yellow removed, G is green and B is blue.

So 147000=210RYGB.
RYGB = 700

So all we need are 4 numbers that multiply together to equal 700. There are a lot of possibilities. Maybe 0 red, 0 yellow, 0 green and 350 blue were removed. If my equation is correct in fact all answers work except C (3 red beads) because 700 is not evenly divisible by 3.

The fact that 7 divides into 147000 exactly twice is meaningless if, for example, 7 yellow beads were removed, or 49 blue.

So why does it HAVE to be 2 exactly?

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Reworked

by sssssss » Sat Apr 11, 2009 8:37 pm
OK, I see that it can't be 0 because the product of point values is 147 000. So they must have removed at least one of each bead. But I still get viable answers for 2,4, and 5 red beads removed.

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by sanju09 » Mon Apr 13, 2009 1:36 am
If the product of the point values of the removed beads is 147,000, this means that at least one of each type of beads were have been surely drawn out, as you can see that the product 147,000 contains all 7,5,3, and 2; when we factor 147,000 to its primes we get it appear like 2^3*3^1*5^3*7^2.

Hence 3 blue beads, 1 green bead, 3 yellow beads, and [spoiler]2 RED[/spoiler] beads were drawn :)
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by abcdefg » Sat Jun 20, 2009 8:04 am
guys I had lots of trouble with this problem too. Please let me know if my understanding is correct.

Initially I also wrote the equation as the following steps:
1. (7r)(5y)(3g)(2b)=147,000
2. 7rygb = 4900
3. rygb= 700
4. r = 700/ygb
I also couldnt figure out why r had to be 2.

But now that I think about it, I actually wrote the equation wrong. The equation should be: (7^r)(5^y)(3^g)(2^b)=147,000. Each bead value is multiplied by the other. That's why we use prime factorization.

Is my understanding of this problem correct? Very misleading indeed. Thanks!

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by Thouraya » Tue Mar 01, 2011 7:13 am
This might be a silly question, but why can't we treat this problem as a ratios problem?

R:Y:G:B
7:5:3:2

their product is 210, and we removed 410,000, so 410,000/210=700 (ok i know there's something that doesn't make sense here), then i divide 700/7 and i get a 100

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by Stuart@KaplanGMAT » Tue Mar 01, 2011 11:29 am
Thouraya wrote:This might be a silly question, but why can't we treat this problem as a ratios problem?

R:Y:G:B
7:5:3:2

their product is 210, and we removed 410,000, so 410,000/210=700 (ok i know there's something that doesn't make sense here), then i divide 700/7 and i get a 100
Hi,

you can't set it up as a ratio because nowhere does it say that you have to pick the colours in equal proportion (in fact, based on the numbers, they're not chosen proportionally).
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