2015 Cayley Problem 17

For the first 30 minutes of a trip, Deanna drove at a constant speed. For the next 30 minutes, she drove at a constant speed 20 km/h faster than her original speed. If the total distance that she travelled was 100 km, how fast did she drive for the first 30 minutes?

\textbf{(A)}\ 80 \text{ km/h}\quad \textbf{(B)}\ 90 \text{ km/h}\quad \textbf{(C)}\ 100 \text{ km/h}\quad \textbf{(D)}\ 110 \text{ km/h}\quad \textbf{(E)}\ 120 \text{ km/h}

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.