What do numbers 1 through 250 added together equal?

1 + 2 + ... + 250 = 31375

1 + 2 + ... + 250 = 1/2 (1 + 2 + ... + 250 + 250 + 249 + ... + 1)

= 1/2 ((1 + 250) + (2 + 249) + ... + (250 + 1))

= 1/2 (251 + 251 + ... + 251)

= 1/2 x 250 x 251

= 31375