Problem Solving

Hello, I need help with this question.
Q: In the airthmetic sequence t1, t2, t3....tn...t1=23. tn=tn-1-3 for each n>1. What is the value of n when tn=-4?

a) -1 b)7 c)10 d)14 e)20

Thanks!!

t1=23 t2=20 and so on...
and tn=t1+(n-1)*d where diffrence is t2-t1=-3
so we have -4=23-3*(n-1) => 3*(n-1)=27 so n=10

IMO10

The Q tells us that each term is 3 less than the previous term, so we can literally list out terms from 23 (the first term) until we get to -4. the correct answer is C.

Because there are only a few terms, it wouldn't take more than 45 seconds to just list terms (23, 20, 17...) until we get to -4.

This is QID 1133

-Patrick