You can write the fraction 0.0003840 a number of ways.
0.0003840 = 0.003840/10 = 0.03840/100 = 0.3840/1000 = 3.840/10000
The one we're interested in is the last one, which has one non-zero digit to the left of the decimal point. The power of 10 in the denominator is the 4th power (the number of zeros is 4). 10^4 in the denominator is equivalent to multiplying by 10^(-4) in the numerator.
Expressed in scientific notation,
[tex]0.0003840=3.840 \cdot 10^{-4}[/tex]
