The directions say to pick a two digit number add together both digits and then subtract the total from your original number. The example they give is 23: 2+3=5, 23-5=18
Here are all of the possible combinations
Here's all of the possible numbers, alongside the calculations:
| 10 | 1 + 0 = 1 | 10 - 1 = 9 |
| 11 | 1 + 1 = 2 | 11 - 2 = 9 |
| 12 | 1 + 2 = 3 | 12 - 3 = 9 |
| 13 | 1 + 3 = 4 | 13 - 4 = 9 |
| 14 | 1 + 4 = 5 | 14 - 5 = 9 |
| 15 | 1 + 5 = 6 | 15 - 6 = 9 |
| 16 | 1 + 6 = 7 | 16 - 7 = 9 |
| 17 | 1 + 7 = 8 | 17 - 8 = 9 |
| 18 | 1 + 8 = 9 | 18 - 9 = 9 |
| 19 | 1 + 9 = 10 | 19 - 10 = 9 |
| 20 | 2 + 0 = 2 | 20 - 2 = 18 |
| 21 | 2 + 1 = 3 | 21 - 3 = 18 |
| 22 | 2 + 2 = 4 | 22 - 4 = 18 |
| 23 | 2 + 3 = 5 | 23 - 5 = 18 |
| 24 | 2 + 4 = 6 | 24 - 6 = 18 |
| 25 | 2 + 5 = 7 | 25 - 7 = 18 |
| 26 | 2 + 6 = 8 | 26 - 8 = 18 |
| 27 | 2 + 7 = 9 | 27 - 9 = 18 |
| 28 | 2 + 8 = 10 | 28 - 10 = 18 |
| 29 | 2 + 9 = 11 | 29 - 11 = 18 |
| 30 | 3 + 0 = 3 | 30 - 3 = 27 |
| 31 | 3 + 1 = 4 | 31 - 4 = 27 |
| 32 | 3 + 2 = 5 | 32 - 5 = 27 |
| 33 | 3 + 3 = 6 | 33 - 6 = 27 |
| 34 | 3 + 4 = 7 | 34 - 7 = 27 |
| 35 | 3 + 5 = 8 | 35 - 8 = 27 |
| 36 | 3 + 6 = 9 | 36 - 9 = 27 |
| 37 | 3 + 7 = 10 | 37 - 10 = 27 |
| 38 | 3 + 8 = 11 | 38 - 11 = 27 |
| 39 | 3 + 9 = 12 | 39 - 12 = 27 |
| 40 | 4 + 0 = 4 | 40 - 4 = 36 |
| 41 | 4 + 1 = 5 | 41 - 5 = 36 |
| 42 | 4 + 2 = 6 | 42 - 6 = 36 |
| 43 | 4 + 3 = 7 | 43 - 7 = 36 |
| 44 | 4 + 4 = 8 | 44 - 8 = 36 |
| 45 | 4 + 5 = 9 | 45 - 9 = 36 |
| 46 | 4 + 6 = 10 | 46 - 10 = 36 |
| 47 | 4 + 7 = 11 | 47 - 11 = 36 |
| 48 | 4 + 8 = 12 | 48 - 12 = 36 |
| 49 | 4 + 9 = 13 | 49 - 13 = 36 |
| 50 | 5 + 0 = 5 | 50 - 5 = 45 |
| 51 | 5 + 1 = 6 | 51 - 6 = 45 |
| 52 | 5 + 2 = 7 | 52 - 7 = 45 |
| 53 | 5 + 3 = 8 | 53 - 8 = 45 |
| 54 | 5 + 4 = 9 | 54 - 9 = 45 |
| 55 | 5 + 5 = 10 | 55 - 10 = 45 |
| 56 | 5 + 6 = 11 | 56 - 11 = 45 |
| 57 | 5 + 7 = 12 | 57 - 12 = 45 |
| 58 | 5 + 8 = 13 | 58 - 13 = 45 |
| 59 | 5 + 9 = 14 | 59 - 14 = 45 |
| 60 | 6 + 0 = 6 | 60 - 6 = 54 |
| 61 | 6 + 1 = 7 | 61 - 7 = 54 |
| 62 | 6 + 2 = 8 | 62 - 8 = 54 |
| 63 | 6 + 3 = 9 | 63 - 9 = 54 |
| 64 | 6 + 4 = 10 | 64 - 10 = 54 |
| 65 | 6 + 5 = 11 | 65 - 11 = 54 |
| 66 | 6 + 6 = 12 | 66 - 12 = 54 |
| 67 | 6 + 7 = 13 | 67 - 13 = 54 |
| 68 | 6 + 8 = 14 | 68 - 14 = 54 |
| 69 | 6 + 9 = 15 | 69 - 15 = 54 |
| 70 | 7 + 0 = 7 | 70 - 7 = 63 |
| 71 | 7 + 1 = 8 | 71 - 8 = 63 |
| 72 | 7 + 2 = 9 | 72 - 9 = 63 |
| 73 | 7 + 3 = 10 | 73 - 10 = 63 |
| 74 | 7 + 4 = 11 | 74 - 11 = 63 |
| 75 | 7 + 5 = 12 | 75 - 12 = 63 |
| 76 | 7 + 6 = 13 | 76 - 13 = 63 |
| 77 | 7 + 7 = 14 | 77 - 14 = 63 |
| 78 | 7 + 8 = 15 | 78 - 15 = 63 |
| 79 | 7 + 9 = 16 | 79 - 16 = 63 |
| 80 | 8 + 0 = 8 | 80 - 8 = 72 |
| 81 | 8 + 1 = 9 | 81 - 9 = 72 |
| 82 | 8 + 2 = 10 | 82 - 10 = 72 |
| 83 | 8 + 3 = 11 | 83 - 11 = 72 |
| 84 | 8 + 4 = 12 | 84 - 12 = 72 |
| 85 | 8 + 5 = 13 | 85 - 13 = 72 |
| 86 | 8 + 6 = 14 | 86 - 14 = 72 |
| 87 | 8 + 7 = 15 | 87 - 15 = 72 |
| 88 | 8 + 8 = 16 | 88 - 16 = 72 |
| 89 | 8 + 9 = 17 | 89 - 17 = 72 |
| 90 | 9 + 0 = 9 | 90 - 9 = 81 |
| 91 | 9 + 1 = 10 | 91 - 10 = 81 |
| 92 | 9 + 2 = 11 | 92 - 11 = 81 |
| 93 | 9 + 3 = 12 | 93 - 12 = 81 |
| 94 | 9 + 4 = 13 | 94 - 13 = 81 |
| 95 | 9 + 5 = 14 | 95 - 14 = 81 |
| 96 | 9 + 6 = 15 | 96 - 15 = 81 |
| 97 | 9 + 7 = 16 | 97 - 16 = 81 |
| 98 | 9 + 8 = 17 | 98 - 17 = 81 |
| 99 | 9 + 9 = 18 | 99 - 18 = 81 |
9, 18, 27, 36, 45, 54, 63, 72 & 81 are the only possible solutions and they always have the same symbol.