Answer:
We could think of the Earth as a sphere, where the diameter of this sphere will the mean diameter between the measure from the North pole to the Soth pole and the measure from opposite points of the equator.
Remember that the mean between two numbers A and B is:
Mean = (A + B)/2
In this case, the mean diameter will be:
D = (7,899.83mi + 7,926.41mi)/2 = 7,913.115 mi
And we know that the surface of a sphere of diameter D is:
S = 4*pi*(D/2)^2
Where pi = 3.14
Then the surface of the Earth will be something like:
S = 4*3.14*(7,913.115mi/2)^2 = 196,618,601.47 mi^2
b) We know that 75% of the Earth's surface is water, now we will use the diameter 7,926.41 mi to compute the surface.
First, the total surface of the Earth will be:
S = 4*3.14*(7,926.41mi/2)^2 = 197,279,843.03 mi^2
And 75% of this is water, then the total water surface is 0.75 times that area, this is:
water surface = 0.75*197,279,843.03 mi^2 = 147,959,882.27 mi^2
