Answer
Total DNA length of US population is 5.1 x $10^{5}$ light-years.
Work Step by Step
First we find the total length of DNA of US population:
total DNA length: = 1 x$10^{10}$miles x 300 million
total DNA length: = 1 x$10^{10}$miles x 3 x $10^{8}$
total DNA length: = 3 x$10^{18}$miles
We have to express this length in m and then into lightyears.
To express this length m, we know that 1 mi = 1.609 x $10^{3}$ m
total DNA length (in m) = 3 x$10^{18}$mi x $\frac{1.609\times10^{3}(m)}{1 (mi)}$
total DNA length (in m) = 4.8x$10^{21}$ m
Now we convert to light-years :
total DNA length (in light-years) = 4.8x$10^{21}$ m x $\frac{1 (light-year)}{(9.46\times10^{15})m}$
total DNA length (in light-years) = 0.51 x $10^{6}$ light-years
total DNA length (in light-years) = 5.1 x $10^{5}$ light-years
