Условие:

9 + 91
8 + 81
7 + 71
...
 
199 + 1
288 + 2
377 + 3
...
 
90 + 91
80 + 81
70 + 71
...

9 + 91 = 181 = 17
8 + 81 = 161 = 15
7 + 71 = 141 = 13
6 + 61 = 121 = 11
 
199 + 1 = 10 + 1 = 11
288 + 2 = 20 + 2 = 22
377 + 3 = 30 + 3 = 33
466 + 4 = 40 + 4 = 44
 
90 + 91 = 991 = 98
80 + 81 = 881 = 87
70 + 71 = 771 = 76
60 + 61 = 661 = 65