Inquiry about Question 13 from "(I'm Overwhelmed) Arithmetic Progress Quiz #3" [6825]

GRE GRE Quant Problem Solving
1

https://www.gregmat.com/quizzes/quiz/im-overwhelmed-arithmetic-progress-quiz-3 (Question 13)

Why is it not 77? If I use the equation (last – first)/multiple + 1 then I get 77

2

That is probably because you’re adding 1001.

First = 13

Last = 988.

Try again using these numbers.