Question 28323
Subtract 3 from 7, because the exchange is always the same. That leaves 4 digits
the number of possible digits is 10 namely, 0,1,2,3,4,5,6,7,8,9
Each of these 10 possible digits can combine with the second digit's 10 possibilities in 10 x 10 different ways. These 10 x 10 possibilities can
combine with the third digit in 10 x 10 x 10 different ways. 
These 10 x 10 x 10 possibilities can
combine with the fourth digit in 10 x 10 x 10 x 10 different ways.
So the answer is {{{10^4}}}