Hey there! :D
So, we need to find how many miles would be traveled per hour in the 85 miles in 3 hours time frame. 
85/3= 28 1/3
So, we could represent this like a sequence of numbers and find the mean. (which is just the average) 

Add all the numbers together. 

Since this was in a span of four hours. 1+3=4, we can divide this by 4 to find the average. 

The average was 34.75 miles per hour. 
I hope this helps!
~kaikers