Question

Based upon the 2010 census data, the total population of the U.S. is 308,745,538 people. Approximately, what is this population in scientific notation

Answer 1

imagine a decimal point after the last digit and, moving it to the left, count how many digits you pass over till you com to the first digit 3.
if this count = n then what you require is
N.NN * 10^n

where N.NN is the population value correct to third digit

Answer 2

stop the count just before the first digit

Answer 3

oh! perhaps you got bored..