2005 Cayley Problem 8

Jack went running last Saturday morning. He ran the first 12 km at 12 km/h and the second 12 km at 6 km/h. Jill ran the same route at a constant speed, and took the same length of time as Jack. Jill's speed in km/h was

\textbf{(A)}\ 8\quad \textbf{(B)}\ 9\quad \textbf{(C)}\ 6\quad \textbf{(D)}\ 12\quad \textbf{(E)}\ 24

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Cayley contests here.