Volume of a sphere with a diameter of 15?

Question

I'm having some trouble with this problem.

The volume of a sphere with a diameter of 15. Round answer to nearest tenth.

 

If the formula for the volume of a sphere is 4/3 x 3.14 x r^3 

Wouldn't the radius be 7.5?

 

So, the answer should be 4/3 x 3.14 x 7.5^3 = 1766.3

 

Why did my teacher say this was incorrect?

Answer 1

If you use 3.1416 for pi instead you get 1767.1 to the nearest tenth.