TITLE: The maximum product of two numbers which together contain all non-zero digits exactly once
QUESTION [3 upvotes]: I found the following puzzle:

Use all the digits from 1 to 9 without repeating, to form two numbers such that     their product is maximum. A digit used should be unique across both the numbers. For example, the numbers formed could be 1234 and 56789.

I know that the answer is 9642 and 87531.
It is obvious that digits should form the numbers in descending order.
I've spent so much time trying to prove that:

for the maximum product numbers should be 4-digit and 5-digit
why people use greedy algorithm for solving the puzzle so that they start from the first digit and add one by one other digits?

Thank you.

REPLY [0 votes]: I'll consider the bit about the maximum requiring a 4-digit and a 5-digit number. I'll show you can't do better with a 2-digit times a 7-digit number, and leave the other cases as an exercise. 
So, suppose $a$ is a 2-digit number, and $b$ a 7-digit number. Now consider the 4-digit number $a'$ and the 5-digit number $b'$ you get by chopping off the last two digits of $b$ and appending them to $a$. These two digits form a number, $x$, and we have $$a'=100a+x,\quad b'={b-x\over100}$$ and $$a'b'=\cdots{\rm algebra}\dots=ab+{b-100a-x\over100}$$ Now $b\ge1,000,000$; $100a\le100,000$; $x\le100$ so the fraction in the display is positive and $a'b'\gt ab$.
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