Arithmetic Sequence Question

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RickH: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Thu May 14, 2009 8:09 pm

by RickH » Sun Aug 15, 2010 7:07 pm

Please help me with this problem:

In the arithmetic sequence t1, t2, t3, ..., tn,..., t1 = 23 and tn = tn-1 - 3 for every n>1. What is the value of n, when tn = -4?

Thanks!

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Source: Beat The GMAT — Problem Solving | Sun Aug 15, 2010 7:07 pm

vijaynaik: Senior | Next Rank: 100 Posts; Posts: 74; Joined: Fri Jun 25, 2010 6:33 pm; Thanked: 3 times

by vijaynaik » Sun Aug 15, 2010 10:30 pm

t1 = 23;
t2 = 23 -3 = 20
t3 = 20 -3 = 17 since each number is 3 less than the previous number

sequence is 23 - (n-1)3

23 - 3n +3 = -4;
30 = 3n;
n = 10;

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RickH: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Thu May 14, 2009 8:09 pm

by RickH » Mon Aug 16, 2010 4:55 am

Thanks man

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