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# m08#27

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m08#27 [#permalink]

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02 Nov 2008, 14:48
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Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

(A) 16
(B) 20
(C) 25
(D) 32
(E) 35

[Reveal] Spoiler: OA
E

Source: GMAT Club Tests - hardest GMAT questions

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

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Re: m08#27 [#permalink]

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26 Aug 2010, 08:53
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Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

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Re: m08#27 [#permalink]

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26 Aug 2010, 11:30
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Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

Comet " B " will appear once in 20 years...
20 * 1 = 20th year
20 * 2 = 40th year
20 * 3 = 60th year

Comet " A " will appear once in 12 years..
12 * 1 = 12th year
12 * 2 = 24th year
12 * 3 = 36th year
12 * 4 = 48th year
12 * 5 = 60th year

When we compare above table, Both the comet will appear again in 60th year from 1979
= 1979+60 =2039th year
current year is 2004
therefore 2039-2004 = 35 years..

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Current Student
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Re: m08#27 [#permalink]

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04 Nov 2008, 08:36
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GMAT TIGER wrote:
fresinha12 wrote:
Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16
* 20
* 25
* 32
* 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

C. 25

NOPE..

so it takes 60 years..so from 1979--60 years is 2039..so when we see it next will be 2039-2004 or 35 yrs from now..

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Re: m08#27 [#permalink]

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02 Sep 2011, 07:29
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E, When both appear back must have total years divisible by 12 & 20, what is multiple of 20 divisible by 12 is 60

1979+60= 2039
Today is 2004 to years to come 2039-2004 = 35 years

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Re: m08#27 [#permalink]

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06 Sep 2011, 12:56
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ameyaberi wrote:
Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

Kudos.

This is how I calculated mine although I don't think I would be able to explain it as well as you.

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Re: m08#27 [#permalink]

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04 Nov 2008, 10:21
Thanks a lot. +1. We'll edit the wording of this one.
fresinha12 wrote:
Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16
* 20
* 25
* 32
* 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

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Re: m08#27 [#permalink]

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26 Aug 2010, 06:52
So... I get that 2039 - 1979 equals 35 but how do you recognize the multiplication of 3 * 2 * 2* 5 to give you the years.

I did separate the 12 and 20 yrs in Greatest Common Factor and Least Common Multiple (4)*(15) but I don´t understand why the multiplication of both will give me the answer of when both comets will see each other again.

Please explain!

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Re: m08#27 [#permalink]

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26 Aug 2010, 08:26
Can u plz explain the correct ans.?

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Re: m08#27 [#permalink]

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26 Aug 2010, 09:28
Hi, how do you that this kind of question needs to be solved with LCM?
I would like a reasonable explanation if possible please.

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Re: m08#27 [#permalink]

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26 Aug 2010, 17:00
Answer is E

LCM of 12 and 20 is 60 hence they would meet in 2029.i.e. 2029-2004 = 35

Why LCM......Lcm can also be explained as occurrence of different digits after some interval so that the result is same.
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Re: m08#27 [#permalink]

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27 Aug 2010, 04:15
Ans:E,
LCM of 12 & 20.

One query:
Do we get such easy questions in GMAT?

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Re: m08#27 [#permalink]

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27 Aug 2010, 21:16
Its E

12 - 20
24 - 40
36 - 60
48 - 80
60 - 100

Its at 60 years, both of them appear together..
1979 + 60 yr = 2039
2039 - 2004 = 35 years

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Re: m08#27 [#permalink]

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30 Aug 2010, 09:13
FN wrote:
Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

(A) 16
(B) 20
(C) 25
(D) 32
(E) 35

[Reveal] Spoiler: OA
E

Source: GMAT Club Tests - hardest GMAT questions

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

LCM is 60, 1979+60 = 2039-2004 = 35, IMO E

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Re: m08#27 [#permalink]

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31 Aug 2010, 05:59
LCM of 12 and 20 is 60. Hence 60 years after 1979 these will be seen again.
Now it is 2004. So it is alrady 25 years since 1979. So additional 60-25 = 35 Years will be needed.
Hence "E"

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Re: m08#27 [#permalink]

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03 Sep 2011, 07:54
Personally, I think the quickest way to solve this problem is to find a common multiple of 12 and 20, viz., 60.

Add 60 years to 1979, viz., 2039. 2039 - 2004 = 35 years.

Should pace under 40 secs this way.
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Re: m08#27 [#permalink]

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06 Sep 2011, 15:39
ameyaberi wrote:
Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

I got E as well. But, it took a while to arrive at the answer.

I know LCM, but didn't know that we can use it for this problem. Can you please explain any other problem scnerios where LCM can used to solve a problem.

Waiting for the day when I will be able to approach and solve maths the way you guys do it! truly commendable!

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Re: m08#27 [#permalink]

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31 Aug 2012, 05:14
Both comets have different frequencies i.e. 12 yrs and 20 yrs.
If B covers 20 yrs A will be covering 12 yrs
At the end of B's 20 yrs , A's 24 yrs
....
.......
in 60 yrs they will be visible together !!!

OR

LCM 12, 20 = 60 MEANS that both are visible together at every 60 years.
In 1979 both appeared together, so 60 yrs after again they will be visible together, i.e in 2039.
Now it is 2004 so in another 35 years it will be seen together again.

(E) Wins
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Re: m08#27 [#permalink]

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17 Aug 2014, 23:48
Hi,

The answer will be E (35 Years)

Explanation:-

Comet A is seen every 12 years and Comet B is seen every 20 years. So minimum after 60 years (smallest number to be divisible by both 12 and 20) both comets will be seen together.

Last observed together in 1979. So will be observed together again in 1979+60 = 2039
As current year is given as 2004, we have to wait 2039-2004=35 years more to see both comets together again.

Thanks.
DR

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Re: m08#27   [#permalink] 17 Aug 2014, 23:48
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