Everyone knows that 3 x 7 is 21, but how many people know what 333 x 777 is, without using a calculator? (It is 258,741 by the way.)
If you double that number you get 517,482. Do you notice anything similar about those two numbers? They have the same digits. And I have yet to find another number that, when doubled, gives you the exact same digits, just in a different order.
Play around with a calculator and see if you can find any other numbers like that. Or not.