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t1, t2, t3, t4...tn what is the value of n? 1) Sum of n

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t1, t2, t3, t4...tn what is the value of n? 1) Sum of n [#permalink] New post 09 Jan 2007, 06:43
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t1, t2, t3, t4...tn what is the value of n?

1) Sum of n terms is 3,124

2) Mean of terms is 4


I got a 47 on the prep and these are the dumb ones I get wrong. How to proceed?

Thanks
Eric
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 [#permalink] New post 09 Jan 2007, 07:17
(C) for me :)

Stat 1
We could have any value for n.

o If n = 1, then Sum t(n) = 3,124 = t(1)
o If n = 100 as well as t(1) = 3,124 and t(k) = 0 with 1 < k <= n, Sum t(n) = 3,124

INSUFF.

Stat 2
Here again, we could have any value of n and get an averge of 4.

INSUFF.

Both (1) & (2)

The rule : Sum / n = av gives us the answer.

n = Sum / av = 3124 / 4 = 781.

SUFF.

Last edited by Fig on 09 Jan 2007, 07:50, edited 1 time in total.
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 [#permalink] New post 09 Jan 2007, 07:45
Thanks. Looks awfully easy now. Number sequences are hard for me. Not sure why. The math behind them is pretty easy.

Eric
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 [#permalink] New post 09 Jan 2007, 07:54
Yes :).... Usually, the used operands are + or - and *. So we are never so far in the concept of number sequence for the GMAT. :)... Pretty good news ;)

I think it's a question of practice to carck and to feel confortable with these questions :)

ericttz wrote:
Thanks. Looks awfully easy now. Number sequences are hard for me. Not sure why. The math behind them is pretty easy.

Eric
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 [#permalink] New post 09 Jan 2007, 08:29
Fig wrote:
(C) for me :)

Stat 1
We could have any value for n.

o If n = 1, then Sum t(n) = 3,124 = t(1)
o If n = 100 as well as t(1) = 3,124 and t(k) = 0 with 1 < k <= n, Sum t(n) = 3,124

INSUFF.

Stat 2
Here again, we could have any value of n and get an averge of 4.

INSUFF.

Both (1) & (2)

The rule : Sum / n = av gives us the answer.

n = Sum / av = 3124 / 4 = 781.

SUFF.


Hi Fig,

can we assume that it is Arithmetic progression?
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 [#permalink] New post 09 Jan 2007, 08:35
aurobindo wrote:
Fig wrote:
(C) for me :)

Stat 1
We could have any value for n.

o If n = 1, then Sum t(n) = 3,124 = t(1)
o If n = 100 as well as t(1) = 3,124 and t(k) = 0 with 1 < k <= n, Sum t(n) = 3,124

INSUFF.

Stat 2
Here again, we could have any value of n and get an averge of 4.

INSUFF.

Both (1) & (2)

The rule : Sum / n = av gives us the answer.

n = Sum / av = 3124 / 4 = 781.

SUFF.


Hi Fig,

can we assume that it is Arithmetic progression?


No. We are never allowed to assume something that restricts the original problems :). We must remain general if the problem is general :)

It could be any kind of sequences here :).

o t(n) = t(n-1) + 1
o t(n) = t(n-1)
o t(n) = 2*t(n-1)
o t(n) = 1/t(n-1)
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 [#permalink] New post 09 Jan 2007, 09:25
Fig wrote:
aurobindo wrote:
Fig wrote:
(C) for me :)

Stat 1
We could have any value for n.

o If n = 1, then Sum t(n) = 3,124 = t(1)
o If n = 100 as well as t(1) = 3,124 and t(k) = 0 with 1 < k <= n, Sum t(n) = 3,124

INSUFF.

Stat 2
Here again, we could have any value of n and get an averge of 4.

INSUFF.

Both (1) & (2)

The rule : Sum / n = av gives us the answer.

n = Sum / av = 3124 / 4 = 781.

SUFF.


Hi Fig,

can we assume that it is Arithmetic progression?


No. We are never allowed to assume something that restricts the original problems :). We must remain general if the problem is general :)

It could be any kind of sequences here :).

o t(n) = t(n-1) + 1
o t(n) = t(n-1)
o t(n) = 2*t(n-1)
o t(n) = 1/t(n-1)


Thanks Fig. Now I understand ur logic :-D
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Series [#permalink] New post 10 Oct 2010, 21:59
If the terms of sequance are t1, t2, t3.... tn. What is the value of n?
1. The sum of n terms is 3124
2. The average of n terms is 4
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Re: Series [#permalink] New post 10 Oct 2010, 22:02
Re: Series   [#permalink] 10 Oct 2010, 22:02
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