When you divide 6x10^24 by 8x10^8 you end up with [tex]\frac{6}{8} x10^{16}[/tex]
This simplifies to [tex]0.75x10^{16}[/tex]
Now this number is not written in scientific notation as it does not have its first digit before the decimal, so we must move the decimal of this one to the right.
To do that you must multiply the number by 10.
This means that the exponet of 10 must decrease by 1.
This makes the answer [tex]7.5x10^{15}[/tex]
