Query about Question 1 from "(I'm Overwhelmed) Arithmetic Progress Quiz #2" [6731]

GRE GRE Quant Problem Solving
1

https://www.gregmat.com/quizzes/quiz/im-overwhelmed-arithmetic-progress-quiz-2 (Question 1)

Hi, I’m a little confused with the sum of the first # of intervals. I understand the sum of all integers perfectly fine, but I guess when to add up pairs is throwing me off.

So in question 1, the value of each pair is accounted for by adding 1 + 7 = 8. We later use this number of pairs to divide and get 4, but it changes to 3 with a leftover of 4? I’m a little confused by this. Meanwhile, in question 2, the number of pairs isn’t changed and the formula seems more straightforward.

I have a weak quant foundation, so I know for certain the issue is on my end. I guess I just need a more simplified explanation. Thanks!

2

Can you explain what you understand from the solution?

3

Yes, so I made sure to do the quant mountain, so I memorized the formula of Last - First + 1. And for the sum of all integers, I memorized the following formula:

Ex. Sum of all integers from 50 - 90, inclusive.

90 - 50 + 1 = 41

90 + 50 = 140

41 x 140 / 2 = 2870

But for the sum of the first seven integers, such as a determining the pairs, I think I overthink things. Should I write it all out to avoid this confusion with pairs? (Example: 1 + 2 + 3 + 4 + 5 + 6 + 7)

4

Number of integers: 7
(7 + 1) = 8
(7 \times 8)/2 = 28

same thing right?

1 Like
5

Ohhh, so we still use a similar formula! Thank you so much!