Formula for finding the area of a circle:

We need to radius but we only have the diameter.
Luckily for us, if you have the diameter, you have the radius.
Because the radius is the diameter over 2.
10 / 2 = 5
Plug in the values.
A = (3.14)5^2
A = (3.14)25
A = 78.5
So, the answer is 78.5.