How do I add numbers 119 through 220 quickly without a calculator?
Note that we have
119 + 120 + 121 + ..... + 218 + 219 + 220
We can rearrange this as
(119 + 220 ) + (120 + 219) + ( 121 + 218) + ........
Note that each pair sums to 339
And we have [ 220 - 119 + 1] / 2 = 51 pairs
So....51 (339) = 17,289
