GMATPREP question

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colors: Newbie | Next Rank: 10 Posts; Posts: 4; Joined: Fri Nov 10, 2006 11:20 am

by colors » Sun Dec 24, 2006 10:01 am

Hello, I need help with this question.
Q: In the airthmetic sequence t1, t2, t3....tn...t1=23. tn=tn-1-3 for each n>1. What is the value of n when tn=-4?

a) -1 b)7 c)10 d)14 e)20

Thanks!!

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Source: Beat The GMAT — Problem Solving | Sun Dec 24, 2006 10:01 am

thankont: Senior | Next Rank: 100 Posts; Posts: 41; Joined: Fri Dec 15, 2006 9:23 amFollowed by: 1 member; Location: Greece

by thankont » Mon Dec 25, 2006 12:34 am

t1=23 t2=20 and so on...
and tn=t1+(n-1)*d where diffrence is t2-t1=-3
so we have -4=23-3*(n-1) => 3*(n-1)=27 so n=10

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Haldiram Bhujiawala: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Thu Mar 11, 2010 3:23 am; Thanked: 1 times

by Haldiram Bhujiawala » Fri May 21, 2010 8:29 am

IMO10

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Patrick_GMATFix: GMAT Instructor; Posts: 1052; Joined: Fri May 21, 2010 1:30 amFollowed by: 98 members; Thanked: 335 times

by Patrick_GMATFix » Fri May 21, 2010 9:48 am

The Q tells us that each term is 3 less than the previous term, so we can literally list out terms from 23 (the first term) until we get to -4. the correct answer is C.

Because there are only a few terms, it wouldn't take more than 45 seconds to just list terms (23, 20, 17...) until we get to -4.

This is QID 1133

-Patrick

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