If you move the decimal point left until it is just to the right of the most-significant digit, the number of places you moved it is the power of ten that your new number is multiplied by.
[tex]400,000=4\times 10^{5}\\200=2\times 10^{2}[/tex]
Then multiplication can proceed in the usual way, taking into account the rules of exponents.
[tex]4\cdot 10^{5}\times 2\cdot 10^{2}=8\cdot 10^{(5+2)}\\\\=8\cdot 10^{7}[/tex]
