203 + 210 + 217 + ... + 392 + 399 =
= 7(29 + 30 + 31 + ... + 56 + 57)
= 7(1 + 28 + 2 + 28 + 3 + 28 + ... + 28 + 28 + 29 + 28)
= 7(1 + 2 + 3 + ... + 28 + 29 + 28 * 29)
= 7(1 + 2 + 3 + ... + 28 + 29 + 812)
= 7 * 812 + 7 * (1 + 2 + ... + 28 + 29)
= 5684 + 7 * (29 * 30)/2
= 5684 + 3045
= 8729
