Area of a circle is πr². (The formula is easy to recall if you just remember the joke--πr²--no, pie are round!)
So we must calculate the difference of the areas to determine what the horse cannot reach.
As we are given the garden's diameter, we must divide it in half to get its radius. The rope, of course, already represent a radius.
π25² - π5² = π(25² - 5²) = π(625 - 25) = π600
= 1885 square meters.
