1+2-3+4+5-6+7+8-9+...+28+29-30=

1+2-3+4+5-6+7+8-9+...+28+29-30=

First find 1 + 2 +. + 30 = 31 * 15 = 465
3 + 6 + 9.. + 30 = 180
The answer is 465-180 * 2 = 105