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Tough GMAT prep CAT question

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by 700goal » Tue Nov 25, 2008 2:05 pm
I ran into this question on the official GMAT CAT that you can download and don't know how to approach it.

In the airithmetic sequence t1, t2, t3..... tn where t1= 23 and tn=tn-3 what is the value of n when tn=-4?

Thanks
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by niraj_a » Tue Nov 25, 2008 3:03 pm
t1 = 23, now reduce 3 from 23 each time i.e.

t2 = 20
t3 = 17

and so on. you will find that t10 = -4 i.e. n = 10
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by parallel_chase » Tue Nov 25, 2008 3:05 pm
You can simply use the AP formula

An = A1 + (n-1)d

-4 = 23 + (n-1)*-3

-4 = 23 -3n + 3

3n = 30

n=10

Hope this helps.
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by niraj_a » Tue Nov 25, 2008 5:13 pm
whats the diff between AP and GP formulas? could you post both of those here?
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