Kate rode her bicycle for 30 minutes at a speed of 16 mph, then walked for 90 minutes at a speed of 4 mph. What was her overall average speed in miles per hour?
$\mathrm{(A)}\ 7\qquad \mathrm{(B)}\ 9\qquad \mathrm{(C)}\ 10\qquad \mathrm{(D)}\ 12\qquad \mathrm{(E)}\ 14$
\[16 \cdot \frac{30}{60}+4\cdot\frac{90}{60}=14\]
\[\frac{14}2=7\Rightarrow\boxed{A}\]