Calculate area of figure 
What is the area of the shaded region of the given $8 \times 5$ rectangle?


Attempt: We know that the area of a kite is $\dfrac{pq}{2}$ where $p$ and $q$ are the diagonals. Thus, since the two lines in the figure intersect at the center of the square, the length of the long diagonal of one of the two shaded kites is half the length of the diagonal of the rectangle, which is $\dfrac{\sqrt{89}}{2}$. Thus the answer should be $\dfrac{\sqrt{89}\sqrt{2}}{2}$ but this is not the right answer (answer is $6.5$). What did I do wrong?

We draw a diagonal from the upper-left to the bottom-right corner. This divides the two quadrilaterals into four triangles, two with base $1$ and height $4$ and two with base $1$ and height $\frac{5}{2}.$ Using basic area calculations, the total area should be $\boxed{6.5}.$
