Use (as first grid) the 5x5 carpet of the 3x3 magic square and (as second grid) the 3x3 carpet of the 5x5 magic square to construct a 15x15 magic square. Take [number -/- 1] x 25 from the first grid and add 1x number from the same cell of the second grid.
25 x [number -/- 1] from 5x5 carpet of a 3x3 magic square
| 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 
| 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 
| 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 
| 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 
| 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 
| 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 
| 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 
| 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 
| 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 
| 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 
| 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 
| 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 
| 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 2 | 9 | 4 | 
| 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 7 | 5 | 3 | 
| 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 6 | 1 | 8 | 
+ 1x number from 3x3 carpet of a 5x5 (pan)magic square
| 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 
| 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 
| 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 
| 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 
| 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 
| 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 
| 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 
| 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 
| 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 
| 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 
| 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 1 | 19 | 7 | 25 | 13 | 
| 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 10 | 23 | 11 | 4 | 17 | 
| 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 14 | 2 | 20 | 8 | 21 | 
| 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 18 | 6 | 24 | 12 | 5 | 
| 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 22 | 15 | 3 | 16 | 9 | 
= 15x15 magic square
| 26 | 219 | 82 | 50 | 213 | 76 | 44 | 207 | 100 | 38 | 201 | 94 | 32 | 225 | 88 | 
| 160 | 123 | 61 | 154 | 117 | 60 | 173 | 111 | 54 | 167 | 110 | 73 | 161 | 104 | 67 | 
| 139 | 2 | 195 | 133 | 21 | 189 | 127 | 20 | 183 | 146 | 14 | 177 | 145 | 8 | 196 | 
| 43 | 206 | 99 | 37 | 205 | 93 | 31 | 224 | 87 | 30 | 218 | 81 | 49 | 212 | 80 | 
| 172 | 115 | 53 | 166 | 109 | 72 | 165 | 103 | 66 | 159 | 122 | 65 | 153 | 116 | 59 | 
| 126 | 19 | 182 | 150 | 13 | 176 | 144 | 7 | 200 | 138 | 1 | 194 | 132 | 25 | 188 | 
| 35 | 223 | 86 | 29 | 217 | 85 | 48 | 211 | 79 | 42 | 210 | 98 | 36 | 204 | 92 | 
| 164 | 102 | 70 | 158 | 121 | 64 | 152 | 120 | 58 | 171 | 114 | 52 | 170 | 108 | 71 | 
| 143 | 6 | 199 | 137 | 5 | 193 | 131 | 24 | 187 | 130 | 18 | 181 | 149 | 12 | 180 | 
| 47 | 215 | 78 | 41 | 209 | 97 | 40 | 203 | 91 | 34 | 222 | 90 | 28 | 216 | 84 | 
| 151 | 119 | 57 | 175 | 113 | 51 | 169 | 107 | 75 | 163 | 101 | 69 | 157 | 125 | 63 | 
| 135 | 23 | 186 | 129 | 17 | 185 | 148 | 11 | 179 | 142 | 10 | 198 | 136 | 4 | 192 | 
| 39 | 202 | 95 | 33 | 221 | 89 | 27 | 220 | 83 | 46 | 214 | 77 | 45 | 208 | 96 | 
| 168 | 106 | 74 | 162 | 105 | 68 | 156 | 124 | 62 | 155 | 118 | 56 | 174 | 112 | 55 | 
| 147 | 15 | 178 | 141 | 9 | 197 | 140 | 3 | 191 | 134 | 22 | 190 | 128 | 16 | 184 | 
Each random chosen 3x5 or 5x3 rectangle gives the magic sum of 1695.
I have used the method composite, AxB compact to construct
12x12, 15x15, 20x20, 21x21, 24x24, 28x28, 30x30a and 30x30b