Well, 10^3 power is 10*10*10 which would equal 1000.
I would think of it like a fraction.
400/1000 You can single out a 100 for both. so by simplifying you get 4/10 because 400÷100=4 and 1000/100=10. You can simplify this down mkre by dividing two into 4/10 to get 2/5.
Therefore, 400 divided by 10 to the third power is 2/5