Oct 1, 2016

Have a look if you had this in mind:

#### Explanation:

I will try to "manipulate" your fraction to take advantage of scientific notation as:
$\frac{8400}{1.2 \times {10}^{7}} = \frac{84 \times {10}^{2}}{12 \times {10}^{6}} = \frac{84}{12} \cdot {10}^{2} / {10}^{6} =$

as you can see I split the division into two "easy" bits:

$\frac{84}{12} = 7$

${10}^{2} / {10}^{6} = {10}^{2 - 6} = {10}^{-} 4 = \frac{1}{10} ^ 4$
using the properties of exponents.

Finally, grouping them together:

$\frac{8400}{1.2 \times {10}^{7}} = 7 \cdot \frac{1}{10} ^ 4 = 7 \times {10}^{-} 4 = 0.0007$