.
This exercise is to check if a person,
who solves it, has enough common sense.
Knowledge of Math is not necessary.
Simplify the problem and consider to construct two two-digit numbers with minimal product.
It is clear that the tens digits must be as small as it is possible, and different.
So, the tens digits are 1 and 2.
Further, ones digits must be as small as possible and different, but not 1 and 2.
So, the ones digits are 3 and 4.
Now we have two pairs (13,24) and (14,23) to chose from.
I will leave it to you to figure out which of these two pairs gives smallest product.
Now use the same idea to solve the problem for 5-digit numbers.