Physics help please physics general?

1)Global positioning satellites (GPS) can be used to determine positions with great accuracy. The system works by determining the distance between the observer and each of several satellites orbiting Earth. If one of the satellites is at a distance of 22,000 km from you, what percent accuracy in the distance is required if we desire a 6 meter uncertainty?

%

How many significant figures do we need to have in the distance?

2()))) Estimate the number of gallons of gasoline consumed by the total of all automobile drivers in the United States, per year. Assume there are about 300,000,000 people in the United States, half of them have cars, they each drive 11,000 miles per year, and their cars get 20 miles per gallon of gas.

.........? gal/year.

3))) A typical adult human lung contains about 290 million tiny cavities called alveoli. Estimate the average diameter of a single alveolus. Assume the alveoli are spherical and a typical human lung is about 1.9 liters.

....? m