The formulas chapter, problem number 10 at the end, can someone please clarify or rephrase the solution so I can understand what they are saying?

Also for problem number 4, I found the sum of all the numbers in the first set, and subtracted that from the sum of all the numbers in the last set and I got 2820, while the answer is 2800 doing it their way. Is there something that I am missing?

Hi,

Please make sure to post the entire text of the question you are asking about (as well as any answer choices, if applicable). The instructors are trying to get to as many posts as possible in a limited amount of time - if we also had to look up and transcribe all of the questions, we wouldn't be able to get through all of the posts. Thanks!