A man covers half part of a certain distance at 15 km/hr. He covers 40 percent of remaining distance at the speed of 12 km/hr and covers rest of the distance at the speed of 21 km/hr. What is his average speed for the entire journey?
1
15 km/hr
2
164/9 km/hr
3
140/9 km/hr
4
139/9 km/hr