isnt this answer insufficient?

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vini vidi vici: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Sun Oct 05, 2008 8:30 am

by vini vidi vici » Thu Oct 23, 2008 5:25 am

Starting from the beginning of the trail, a one-mile-long walking trail has a red marking every fifth of a mile, a blue marking every fourth of a mile, and green markings at the beginning and end of the trail. There are no other markings on the trail. Which of the following gives all of the possible distances, in fractions of a mile, from one marking on the trail to the next?

A. 1/4 and 1/5 only

B. 1/5 and 1/20 only

C. 1/4, 1/10 and 1/20 only

D. 1/5, 3/10, 1/10 and 1/20

E 1/4, 1/5, 1/10 and 1/20

Answer is D. However, IMO 1/4 should also be included in the answer list as that gives the first distance from the green marking to the blue marking.

a bit confused on this one...

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Source: Beat The GMAT — Problem Solving | Thu Oct 23, 2008 5:25 am

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

Re: isnt this answer insufficient?

by Ian Stewart » Thu Oct 23, 2008 6:28 am

vini vidi vici wrote:Starting from the beginning of the trail, a one-mile-long walking trail has a red marking every fifth of a mile, a blue marking every fourth of a mile, and green markings at the beginning and end of the trail. There are no other markings on the trail. Which of the following gives all of the possible distances, in fractions of a mile, from one marking on the trail to the next?

A. 1/4 and 1/5 only

B. 1/5 and 1/20 only

C. 1/4, 1/10 and 1/20 only

D. 1/5, 3/10, 1/10 and 1/20

E 1/4, 1/5, 1/10 and 1/20

Answer is D. However, IMO 1/4 should also be included in the answer list as that gives the first distance from the green marking to the blue marking.

a bit confused on this one...

But there is a red marking on the trail before the first blue marking - the distance from the start of the trail to the first marking is 1/5. It might be easiest to multiply everything by 20 here - then you have red markings at:

4, 8, 12, 16

and blue markings at

5, 10, 15

So together we have markings at:

0, 4, 5, 8, 10, 12, 15, 16, 20

and the various distances are 1, 2, 3, and 4. Divide by 20 to get the answers (and you have a typo in choice D above- it should say 3/20, not 3/10).

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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vini vidi vici: Newbie | Next Rank: 10 Posts; Posts: 9; Joined: Sun Oct 05, 2008 8:30 am

by vini vidi vici » Fri Oct 24, 2008 1:00 am

thanx a bunch Ian! great help..

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gmat009: Master | Next Rank: 500 Posts; Posts: 458; Joined: Sun Aug 31, 2008 10:44 amFollowed by: 1 member; Thanked: 3 times

by gmat009 » Fri Oct 24, 2008 8:43 am

vini vidi vici wrote:thanx a bunch Ian! great help..

Is there any typo in choice D becoz I am getting 3/10
According to me D is perfectly alright.

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by Ian Stewart » Fri Oct 24, 2008 11:45 am

gmat009 wrote:
vini vidi vici wrote:thanx a bunch Ian! great help..
Is there any typo in choice D becoz I am getting 3/10
According to me D is perfectly alright.

There are red markings every 1/5 of a mile - that is, every 2/10 of a mile - so even if there were no blue markings at all, there could never be a distance of greater than 2/10 between consecutive markings. So yes, 3/10 is a typo.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

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gmat009: Master | Next Rank: 500 Posts; Posts: 458; Joined: Sun Aug 31, 2008 10:44 amFollowed by: 1 member; Thanked: 3 times

by gmat009 » Fri Oct 24, 2008 11:59 am

Ian Stewart wrote:
gmat009 wrote:
vini vidi vici wrote:thanx a bunch Ian! great help..
Is there any typo in choice D becoz I am getting 3/10
According to me D is perfectly alright.
There are red markings every 1/5 of a mile - that is, every 2/10 of a mile - so even if there were no blue markings at all, there could never be a distance of greater than 2/10 between consecutive markings. So yes, 3/10 is a typo.

Ian,
You may be right. May be I am taking distances in wrong way.
I made markings on number line and was trying to find distances.
I was calculating distance between 2nd Blue marking and 1st Red marking
2/4 - 1/5 = 3/10

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