Meg purchased a stereo system at a local store. It cost $675. She paid for it in six equal installments of $139 each. How much interest did she pay?

Explanation plz. Would like to know how to solve for future questions.

Which situation can be modeled by the inequality 5 + 10w ≥ 45? a. you start with $5 and save $10 a week until you have at least $45. b.you start with 5 baseball cards and purchase 10 cards every week until you have at most 45 cards. c.you start with 5 water bottles and purchases cases of 10 water bottles each until you have a total of 45 water bottles. d.you spend $5 plus $10 per week until you have less than $45.