Answer:
approximately 1.213 billion gallons of fuel will be saved per year.
Explanation:
Distance to the store = 1/2 mile
If my car gets 30 miles to one gallons, then for 1/2 mile my car will use
1/2 ÷ 30 = 1/2 x 1/30 = 1/60 gallons
Statistics puts the number of cars users on Earth to be about 1.4 billion = 1.4 x 10^9 cars
If every driver makes this decision once a week for a year, that will be 52 times a year since there are 52 week in a year.
Total gas that will be saved in one week = 1/60 x 1.4 x 10^9 = 23333333.3 gallons
In a year that will be = 23333333.3 gallons x 52 = 1.213 x 10^9 gallons
that is approximately 1.213 billion gallons of fuel saved per year