How to calculate 1+2+3+...+99+100 without a calculator?

Answer 1

1+2+3+...+99+100
form pairs in the following way:
= (1+100) + (2+99) + (3+98) + ....
= 101 + 101 + 101 + ...
how many 101's will you have ?

Legend has it that the famous German mathematician Gauss, did this in his head when his teacher assigned it in early grade school as "busy work" to keep the kids quiet.

Answer 2

Thank you!