Answer:
3825
Step-by-step explanation:
Given that we have to pick digits from the list 1, 2, 3, 4, 5, 6, 7, 8, 9 just once and build 4 digit numbers.
Find the two numbers which have the sums as the smallest
Since when the two numbers formed are the smallest the sum will be the smallest, we can pick digits in such a ways that the numbers have value the smallest. If the 1000th digit is small we would get smaller number. So let us take for first digit 1000th place 1 and second number 1000th place as 2.
Similarly for 100th now available are 3,4...8, 9 so let us take 100th digit as 3 and 5 respectively and so on.
So the numbers would be
1357 and 2468
Sum would be