Answer: [tex]\bold{3\times 10^9}[/tex]
Step-by-step explanation:
[tex]5\ bags \times \dfrac{4\ lbs}{1\ bag}\times \dfrac{300,000,000\ people\ US}{2\ people\ household}=3,000,000,000\\\\\\=3\times 10^9[/tex]
The smallest number of significant digits is 1 digit.