SOLUTION: Use each of the digits 0,1,2,3,4,5. Find the smallest positive 2-digit answer. Note can only be 3-digits subtracted from 3-digits exp. 301-254= 47

Let's let {0,1,2,3,4,5} = {A,B,C,D,E,F}, but we don't know
which letter represents which digit.

We write the subtraction vertically as in basic math.

 ABC
-DEF
----
  XY

X and Y can be any digits.

To keep from getting a first digit in the answer, A and D must differ
by only 1 and we must have to borrow from the A, so that we won't
get a hundred's digit.

The first priority is to make X as small as possible.  Since we must borrow
from A, E must be subtracted from a number larger than any of the digits 1 thru
5.  So to make X as small as possible, we choose E as large as possible.
So E must be 5.  

 ABC
-D5F
----
  XY

The smallest number we could subtract 5 from is 9, and that is possible 
only if B is 0.  So we choose B as 0.

 A0C
-D5F
----
  XY

Then X can be 4 as long as we make sure C is smaller than F, making us
have to borrow from 0 so as to leave it a 9.  And we know we can
make sure of that by picking F larger than C, so we have:

 A0C
-D5F
----
  4Y

The remaining unused digits for A,C,D,F are 1,2,3,4.

The second priority is to make Y to be as small as possible. Since we have to
borrow to make C more than 10, we make C as small as possible and F as large as
possible to make the difference small as possible, so we would like to choose C
as 1 and F as 4, if this is possible.  We see it is possible to choose them
that way because it leave 3 and 2 for A and D respectively, as they must differ
by only 1. So the smallest solution is 47 from this subtraction:

 301
-254
----
  47

Well, what do you know?  That was the example given!

Edwin