(100x + 10 + x) + (100x + 20 + x) + (100x + 30 + x) + ... + (100x + 90 + x ) = 1359
10 + 20 + 30 + ... + 90 + 909x = 1359
10 + 20 + 30 + ... + 90 = 10 × (1 + 2 + 3 + 4 + ... + 9) = 10 × 45 = 450
450 + 909x = 1359
909x = 1359 - 450 = 909
909x = 909 deci x = 1 !
