K. 168. Consider all possible seven-digit numbers formed by using each of the digits 1, 2, 3, 4, 5, 6, 7 once.
a) Show that the difference of any pair of such seven-digit numbers is divisible by 9.
b) How many pairs of different numbers can be selected out of them, such that one is a factor of the other?
Solution (in Hungarian)
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