Answer
360 tiles
Work Step by Step
The tiles are 12 inches by 12 inches, which means 1 foot by 1 foot.
We use an arithmetic sequence with the elements:
$a_1=30$
$d=-1$
There are $n=30-15+1=16$ rows of tiles. Determine the total number of tiles $S_{16}$:
$S_n=\dfrac{n(2a_1+(n-1)d)}{2}$
$S_{16}=\dfrac{16(2(30)+(16-1)(-1))}{2}=\dfrac{16(45)}{2}=360$
