Query about Question 4 from "(I'm Overwhelmed) Arithmetic Progress Quiz #3" [6816]

GRE GRE Quant Problem Solving
1

Link: https://www.gregmat.com/quizzes/quiz/im-overwhelmed-arithmetic-progress-quiz-3 (Question 4)

write your question here…I think i got the right answer, i put 13, the answer is 13.. what happened?