The difference 942-249 is a positive multiple of 7. If a, b, and c are nonzero digits, how many 3-digit numbers abc are possible such that the difference abc - cba is a positive multiple of 7?
142
71
99
20
18
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那么简单看，a-c一定是7的倍数，那么只有【8，1】【9，2】，b可以是1-9
所以18种
因此是 2*9=18种
由于a, b, c 为非零
只能是
a = 9, c = 7
a = 8, c = 1
b为任意1到9
故 9 + 9 = 18