1) Since we want to rewrite it in Scientific Notation, we need to count how many decimal places there are between where it is and the decimal place between 7 and 2
[tex]0.000725=7.25\times10^{-4}[/tex]Note that there are 4 decimal places before the 7, so the exponent is negative.
2) Now, for the other way around here the weight of that blue whale can be rewritten into Standard Notation this way:
[tex]4.2\times10^5=4.2\times100000=4200000[/tex]Or we can count ahead and add 5 zeros moving the dot to the right.
3) Hence, the answer is:
[tex]\begin{gathered} a)7.25\times10^{-4} \\ b)4200000 \end{gathered}[/tex]