Cindy leaves school at the same time every day. If she cycles at 20\:km/h, she arrives home at 4:30 in the afternoon. If she cycles at 10\:km/h, she arrives home at 5:15 in the afternoon. At what speed, in km/h, must she cycle to arrive home at 5:00 in the afternoon?
Answer Choices
A. 16\frac23
B. 15
C. 13\frac13
D. 12
E. 18\frac34