A sequence is defined by a_1 = 100 and a_n = a_n-1 - 7. What is a_5?

Each term is $7$ less than the one before it. Starting at $a_1=100$: $a_2=93$, $a_3=86$, $a_4=79$, and $a_5=72$, choice B. As a formula, $a_5=100-4(7)=72$, because four subtractions of $7$ separate the first and fifth terms. Choice C ($79$) is $a_4$, one term short. Choice D ($86$) is $a_3$. Choice A ($65$) is $a_6$, one term too far. Counting terms carefully matters: the fifth term follows four steps, not five. Subtracting repeatedly is simple but easy to miscount.