Question

Find the difference between the sum of all even numbers and the sum of all odd numbers from 0 through 1000.

Answer 1

Think of it this way for eery even number there is an odd number that +1 it so if you find the difference between those it is 1 
(EX: 1  and 2) (EX: 1+3 and 2+4) 
so there are 500 odd numbers so when you add it up the evens add up 500 more