Arthur is driving to David’s house intending to arrive at a certain time. If he drives at 60 ~\mathrm{km} / \mathrm{h}, he will arrive 5 minutes late. If he drives at 90 \mathrm{~km} / \mathrm{h}, he will arrive 5 minutes early. If he drives at n \mathrm{~km} / \mathrm{h}, he will arrive exactly on time. What is the value of n?