Answer:
 4.8×10^10 gallons
Step-by-step explanation:
The desired number is the product of the number of households and the volume of water used by each:
 (1.2×10^8 households)(4×10^2 gal/day/household) = (1.2·4 × 10^(8+2)) gal/day
 = 4.8×10^10 gal/day