A student travels to his school at a speed of 4 km/hour and reaches the school 15 minutes late. On travelling at a speed of 6 km/hour; he reaches the school 5 minutes early. At what speed must he travel to reach the school just in time?
1
41/3 km/hr
2
31/3 km/hr
3
51/3 km/hr
4
61/3 km/hr