Hey there mate!
So, when doing this kind of notation (scientific notation), this is pretty much like braking down the number,and trying to make it more simple.

I hope this helps you! :)
25% = 1/4
So to get the unknown number, multiply 9 by 4:
9 × 4 = 36
Im not for sure if i did this right but my calculator says that its 420.
hope i helped.