a+b+c+d = 27
abcd + dcba =
= 1000a + 100b + 10c + d + 1000d + 100c + 10b + a =
= 1001a + 110b + 110c + 1001d =
= 11×(91a + 10b + 10c + 91d) =
= 11×(81a+10a+ 10b + 10c + 10d + 81d) =
= 11×[81×(a+d) + 10×(a+b+c+d)] =
= 11×[3×27×(a+d) + 10 × 27] =
= 11×[27×(3a+3d+10)] =
= 296 × (3a+3d+10) se divide cu 297
