Lesson: General GRE Math Strategies

I'm confused on the last problem. How do we know that there are 21 pairs that add to 65? Why is 53-12 relevant, and why does y-x+1 factor in?

you are my god. i love you man. 

I've never had problems with math so I didn't think I needed any help with GRE math since it's basically high school math. But you've got some really nice tips here man. Thanks!

I find it more intuitive for this type of problem to find the average of the series and then multiply that by the number of values in the series to get the sum. For example, first add the first value in the series and the last value in the series and then divide the sum by 2. This gives me the average value (or middle value) of the series. In this case (12+53)/2 = 32.5. Then, I multiply this value by the number of values in the series to get the sum of the series.