Its a AP series.

Term = a + (n-1)d
where a = first term
n = nth term
d = difference

So here a =23, d=-3 and we need to find the nth term whose value is -4.

23+ (n-1)*-3 = -4
-3n+3 = -27
n = 10

Is the OA 10

Thanks, but I don't understand what you meant here:

Term = a + (n-1)d
where a = first term
n = nth term
d = difference

So here a =23, d=-3 and we need to find the nth term whose value is -4.
How did you know the first term was 23?

Also, whats an AP? Arithmetic progression?

Thanks

gmatrant

I got the same sanswer as u, used the same AP formula :)) but when I saw the ans given by the GMAT prep people it is 14.

Any thoughts?

I used the AP formula and did it manually
tn=t1 + (n-1)d
-4=23+(n-1)(-3)
n=10

23, 20, 17, 14, 11, 8, 5, 2, -1, -4 <----10th term

can't see how 14 is the answer (t1 would have to be 35 for n to be 14).

But I'm still somewhat unclear as to this AP thing, especially this: tn=t1 + (n-1)d
where do I get more information on how to attack this?

thanks!

It's an arithmetic sequence. the question states that each consecutive term is 3 less than the previous.

for instance, the 3rd term would be 3 less than the second term, which is 3 less than the first term (hope thats clear)

1st term = 23
2nd term = 23-3 or 23+(1)(-3) = 20
3rd term = 23-3-3 or 23+(2)(-3) = 17
4th term = 23-3-3-3 or 23+(3)(-3) = 14
and so on...

To rewrite this into a formula, the value of the nth term would be 23+(n-1)(-3)
i.e. Value of nth term = 1st term + (n -1)(difference)
or
Tn = T1 + (n-1)d

https://www.mathcentre.ac.uk/resources/l ... ssions.pdf

refer to this link for more details

OOPS! forgive me! I didn't see that little comma that said T(1)=23!
For some reason, I thought that was a continuation from T(n)...
Now it makes sense, cos I was wondering how you determined that T(1)=23..

Thanks for your help!
(so can this formula be used for any sortof progression question?)

this formula can only be used for addition.
For different types of progressions, check that link that r12007 posted. It's pretty good.

I still don`t get it. The questions says what is the value of N when Tm =-4, what jrbrown has answered is what is the value of the 4th term.