To find the area that the sprinkler can spray, we'll start by determining the area of the semicircle that represents the area covered by the sprinkler as it rotates 180°.
The formula for the area of a circle is:
\[ A = \pi r^2 \]
Since the sprinkler only covers a semicircle (180°), we will take half of the area of the full circle. The radius \( r \) in this case is the distance from the sprinkler head, which is 15 feet.
First, calculate the area of the full circle:
\[ A_{\text{circle}} = \pi (15)^2 = \pi (225) = 225\pi \text{ ft}^2 \]
Now, to find the area of the semicircle:
\[ A_{\text{semicircle}} = \frac{1}{2} A_{\text{circle}} = \frac{1}{2} (225\pi) = 112.5\pi \text{ ft}^2 \]
Thus, the area of the backyard that the sprinkler can spray is:
\[ \boxed{112.5\pi \text{ ft}^2} \]