Quant Flashcard Quiz Answer Explanations?

Are there any explanations of the quant flashcard quizzes posted anywhere? I am struggling with question 15 on the quant flashcard set 1 quiz.

Not sure how to subtract the duplicate factors to arrive at 842.

Which numbers are multiples of both 2 and 3? You need to subtract that from (sum of all multiples of 2 + sum of all multiples of 3).