Which one is bigger: $\;35{,}043 × 25{,}430\,$ or $\,35{,}430 × 25{,}043\;$?

If we have two pairs of numbers, and both pairs add up to the same total, then the pair with the larger product will be the pair that's closer together. So the answer is the first pair.

(This picture might help: if you want a rectangle with a fixed perimeter to have the biggest possible area, you want it to be a square.)

Intuitively, you want the $430$ to multiply the biggest thing it can, which is $35{,}000$, so the first.

No calculation or equation needed at all.

When you have a fixed length to distribute over the 4 borders of a rectangle, and you want to get the maximum possible surface, you will get a square (equal length borders).

In other words, when adding a value to two multiplicants, you will get the biggest result when minimizing the difference of the multiplicants: The bigger one is the equation wher you add 0,43 to the smaller number and 0,043 to the bigger one.