Here's a pattern to consider:
1+100=101
2+99=101
3+98=101
4+97=101
5+96=101
.....
This question relates to the discovery of Gauss, a mathematician. He found out that if you split 100 from 1-50 and 51-100, you could add them from each end to get a sum of 101. As there are 50 sets of addition, then the total is 50×101=5050
So, the sum of the first 100 positive integers is 5050.
Quick note
We can use a formula to find out the sum of an arithmetic series:

Where s is the sum of the series and n is the number of terms in the series. It works for the above problem.
The slope is 1, you can see as it goes right one it goes up one, so the rise/ run would be 1
ANSWER: 1
Make sure you study, and if necessary, do some practice problems. Then, check to make sure you are right, and if not, find out why, and correct your mistakes.
Your sum is 4 times 10, or 40. We want to end up with 40. Subtract 8 because you added 8 before to work backwards. You get 32. Divide by 16 because the opposite of multiplication is division. You would get 2.