How do you calculate 1-2 plus 3-4 plus 5-6---------- plus 19-20?

Answer 1

You have 10 "pairs" of numbers which all give the answer, -1 (negative one or minus one). So your sum is -1 + (-1) + (-1) +... Ten times. i.e (-1) multiplied by 10. So the answer is -10 (negative 10).