The surface area of a sphere is given by the formula:
4πr^2
where r is the radius of the sphere. The diameter of Earth is about 7920 miles, so the radius is approximately 3960 miles.
Substituting this value into the formula, we get:
4π(3960)^2 ≈ 197352720 sq miles
About 70% of this area is water, so we can find the amount of water on Earth by multiplying the total surface area by 0.7:
197352720 * 0.7 ≈ 138146904 sq miles
Therefore, about 138,146,904 square miles of Earth's surface is water.