For example, I was adding (9.99 x 104)+ (9.99 x 105).
I raised the first number's exponent from 4 to 5 by dividing it by 10 once more, thus getting 0.999 and the 105 exponent as a way to reverse it (that's how I see scientific notation, you either divide or multiply by ten successive times and then you have the reverse process as scientific notation, so if I get a positive exponent, it reverts a corresponding exponentiation which has a negative exponent, and the converse is also true).
The problem is that after that, when adding 9.99 with .999 I get 10.989 and my final number is 10.989 x 105, which is not a valid number in scientific notation as far as I know.
PS: sorry for my explanation, it feels a bit confused even to me, but that's the best I could conceive as an explanation to the issue I have.
TL;DR (9.99 x 104)+ (9.99 x 105) = 10.989 x 105, how the hell do I solve it?