If 1 gallon of paint covers 400 square feet, about how many gallons of paint are needed to paint the outside of a spherical tank diameter 20 feet

Given:
Diameter if the spherical tank = 20 ft.
Now,
We know,
radius = diameter / 2
          = 20 / 2
          = 10 ft.
Now, 
The area of the spherical tank = 
        
                                            = 

                                            = 

                                            = 
 
                                            =   

Now,
According to the question,
400 ft² needs 1 gallon of paint
1 ft² needs (1 / 400) gallon of paint
1256.64 ft² needs (1 / 400) * 1256.64 gallon of paint
                        = 3.1416 gallon of paint
                        = 3.14 gallon of paint --------------> rounded

So, we need 3.14 gallon of paint