Query about Question 1 from "PrepSwift: Sum of Multiples in Interval" [3611]

https://prepswift.gregmat.com/quizzes/quiz/prepswift-sum-of-multiples-in-interval (Question 1)


Hi Solved this using Last-First/Multiple +1 method. Since the questions asks sum of multiples of 3 in 1-100. The answer using above method should be 1717. Can you please help me understand the same?

How did you get 1717 exactly?