I am not understanding the following problem and how to solve it using the shortcut from the mini quiz review…

Which of the following is the sum of the 75 largest negative integers and the 78 smallest positive integers?

The shortcut was 76+77+78 = 231 as the answer… but I am not understanding the shortcut at all.

Suppose you were asked to find the sum of the 3 largest negative integers and 6 smallest positive integers. What would you do?