To find the area sprayed by the sprinkler, we first note that the sprinkler activates over a semicircular area since it rotates 180°. The radius of this semicircular area is the distance the water can be thrown, which is 15 feet.
The area \( A \) of a full circle is given by the formula: \[ A = \pi r^2 \] where \( r \) is the radius. For a semicircle, the area will be half of that:
\[ A = \frac{1}{2} \pi r^2 \]
Now, substituting the radius given (15 feet):
\[ A = \frac{1}{2} \pi (15)^2 \] \[ A = \frac{1}{2} \pi (225) \] \[ A = \frac{225}{2} \pi \] \[ A = 112.5 \pi \text{ ft}^2 \]
Thus, the area of the backyard that the sprinkler can spray is
\[ \boxed{112.5\pi \text{ ft}^2} \]