Answer:
Their average rate of speed is 53 miles per hour.
Explanation:
Given : A family on a vacation drives 123 miles in 2 hours then gets stuck in traffic and goes 4 miles in the next 15 minutes. The remaining 191 miles of the trip take 
 hours.
 hours. 
To find : What was their average rate of speed to the nearest tenth of a mile per hour ?
Solution : 
We know, 

Total distance traveled by family on vacation is 
D= 123 miles + 4 miles + 191 miles = 318 miles
Total time taken by family on vacation is 
T= 2 hours + 15 minutes + 
 hours
 hours
T= 2 hours + 
 hours +
 hours + 
 hours
 hours
T= 
 hours
 hours
T= 
 hours
 hours
T= 
 hours
 hours
T= 6 hours
Substitute the value in the formula,
 

 
 miles per hour.
 miles per hour.
Therefore, Their average rate of speed is 53 miles per hour.