I would greatly appreciate help with the following problem.
Thanks!
In the arithmetic sequence t1, t2, t3,......., t(n), t1=23 and t(n)=t(n-1)-3 for each n>1. What is the value of n when t(n)=-4?
A) -1
B) 7
C) 10 - correct
D) 14
E) 20
Amit wrote:In the case where the numbers are farther apart (notice the use of farther vs. further from S.C. studying) I would make an equation to solve.
For example:
t1=23
t2=20
t3=17
...
I know that we subract 3 each time we get the new term, so we need -3n somewhere.
We can say t(n) = 26 - 3n. I used 26 because if I plug in n=1, it works for the first equation.
Using the equation and t(n)=-4,:
-4 = 26 - 3n
3n = 30
n = 10
This may look shorter, but during the GMAT I would suggest to just count downwards because you save the risk of creating an incorrect equation and/or simplifying your equation incorrectly.
Cheers!
EstanisladoM601 wrote:How did you figure out the "-3n"? and why the "26"?