AMC 12 2007a Problem 4

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?

$\mathrm{(A)}\ 7 \mathrm{(B)}\ 9 \mathrm{(C)}\ 10 \mathrm{(D)}\ 12 \mathrm{(E)}\ 14$