Question 1108281
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There are  6*5*4*3 = 360  4-letter words made of 6 letters without repetition,  and


There are  10*9*8*7 = 5040 4-digit numbers made of 10 digits from 0 to 9 without repetition.


In all, there are 650*5040 = 1814400 possible serial numbers.
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