4A --> D
A 为1、2
假定A = 1,D = 4,但 4*D 的个位不是1,所只能取 A = 2.4*D 的个位是 2 D = 8成立
(2000 + 100B + 10C + 8)*4 = 8000 + 100C + 10B + 2
400B + 40C + 32 = 100C + 10B + 2
40B + 4C + 3 = 10C + B
39B = 6C - 3 = 3(2C - 1)
13B = 2C - 1 .能被13整除,所以：
B = 1 .C = 7
原式为：2178*4 = 8712