3*3*2*1 = 18
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Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
there are 4
you take the four digits and multiply 4 by 4 you get 16 digits
You had me until "product." The product of 4 digits can't be prime.
The numbers from 1000 to 9999 are all 4 digits, as are -1000 to -9999. Therefore there are 18,000 four-digit numbers in all.