Area of a circle is calculated by this expression....
A = πR²
What this says is Area is equal to the radius squared times pi (3.14 for all intensive purposes.)
So, we need to find A (area.)
We know the diameter (d) is twice the radius, and we know that the diameter is 10.
10/2 = 5.
The radius is 5.
A = π5²
5² = 5 x 5 = 25
A = π25
3.14 (π) * 25 = 78.5
The area of the circle with the diameter of 10 is 78.5 roughly. This is not exact but for what you are doing it will suffice.