Cátia leaves school every day at the same time and returns home by bicycle. When she cycles at $20 \mathrm{~km} / \mathrm{h}$, she arrives home at $4: 30$ PM. If she cycles at $10 \mathrm{~km} / \mathrm{h}$, she arrives home at $5: 15$ PM. At what speed should she cycle to arrive home at 5: 00 PM?