Answer
$3\space \times\space 10^{6}$ cows were required.
Work Step by Step
From the resources, we can find the milk requirement per year in US $=30\space \times 10^{9}\space kg\space - (1)$
Average dairy cow milk production $=10^{4}\space kg\space -\space (2)$
$(1)/(2)$ we can get,
The no. of dairy cows requirement $=\frac{30\times10^{9}\space kg}{10^{4}\space kg}=30\space \times10^{(9-4)}$
$\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space=3\times10^{6}$
