Compare the images in positions 9, 18, 27, 36, 45, 54, 63, 72 and 81.
Number you thought of - sum of digits = subtract one from the other...
10 - 1 = 9
11 - 2 = 9
12 - 3 = 9
13 - 4 = 9
14 - 5 = 9
15 - 6 = 9
16 - 7 = 9
17 - 8 = 9
18 - 9 = 9
19 - 10 = 9
20 - 2 = 18
21 - 3 = 18
22 - 4 = 18
23 - 5 = 18
24 - 6 = 18
25 - 7 = 18
26 - 8 = 18
27 - 9 = 18
28 - 10 = 18
29 - 11 = 18
30 - 3 = 27
31 - 4 = 27
32 - 5 = 27
33 - 6 = 27
34 - 7 = 27
35 - 8 = 27
36 - 9 = 27
37 - 10 = 27
38 - 11 = 27
39 - 12 = 27
40 - 4 = 36
41 - 5 = 36
42 - 6 = 36
43 - 7 = 36
44 - 8 = 36
45 - 9 = 36
46 - 10 = 36
47 - 11 = 36
48 - 12 = 36
49 - 13 = 36
50 - 5 = 45
51 - 6 = 45
52 - 7 = 45
53 - 8 = 45
54 - 9 = 45
55 - 10 = 45
56 - 11 = 45
57 - 12 = 45
58 - 13 = 45
59 - 14 = 45
60 - 6 = 54
61 - 7 = 54
62 - 8 = 54
63 - 9 = 54
64 - 10 = 54
65 - 11 = 54
66 - 12 = 54
67 - 13 = 54
68 - 14 = 54
69 - 15 = 54
70 - 7 = 63
71 - 8 = 63
72 - 9 = 63
73 - 10 = 63
74 - 11 = 63
75 - 12 = 63
76 - 13 = 63
77 - 14 = 63
78 - 15 = 63
79 - 16 = 63
80 - 8 = 72
81 - 9 = 72
82 - 10 = 72
83 - 11 = 72
84 - 12 = 72
85 - 13 = 72
86 - 14 = 72
87 - 15 = 72
88 - 16 = 72
89 - 17 = 72
90 - 9 = 81
91 - 10 = 81
92 - 11 = 81
93 - 12 = 81
94 - 13 = 81
95 - 14 = 81
96 - 15 = 81
97 - 16 = 81
98 - 17 = 81
99 - 18 = 81
Yeah I know, I'm a spoilsport!