My teacher says everything is there to answer this question but i don't know. someone explain. two travellers spend from 12 o'clock to 6 o'clock walking along a level road, up a hill and back again. their pace is 4 mph on the level, 3 mph uphill, and 6 mph downhill. how far do they walk and at what time do they reach the top of the hill?

We can start with the basics. What we know is that this entire trip took 6 hours. So the travelers were able to walk on the level, uphill, and downhill in a total of 6 hours. 

A level mile takes 1/4 of an hour (15 minutes). An uphill mile takes 1/3 of an hour (20 minutes). A downhill mile takes 1/6 of an hour (10 minutes). So, to go and return over the same mile takes half an hour. This means that in 6 hours, they went 12 miles and came 12 miles back. So they went a total of 24 hours. 

If the 12 miles had been all level, it would have taken approximately 3 hours. It the 12 miles had been all uphill, it would have taken approximately 4 hours. When reaching the peak, 3.5 hours would have been the approximate time it took them to reach the top since it is in between.