What is the average speed, if a person drives for 30.0 min at 80.0 km/h, 12.0 min at 100 km/h, and 45.0 min at 40.0 km.h and spends 15.0 min eating lunch and buying gas? The back of my textbook states that the answer is 52.9, but I don't know how to solve it!!

The way to work this and similar questions out is to simply calculate how far the person has driven in total and how long it took them, Then you can determine the average speed

As you know the answer, here is how to do it:

Convert the time in to fractions of hours and times the fraction by the speed they were driving at:

30 min at 80 km/h = 1/2 hour at 80km/h = 40 miles
12 min at 100km/h = 1/5 hour at 100km/h = 20 miles
45 min at 40km/h = 3/4 hour at 40km/h = 30 miles
Total = 90 miles

Total time:
How long did it take him? 30 min + 12 min + 45 min + 15 min for breaks = 102 minutes

Convert minutes to hours:
102 minutes /60 minutes per hour = 1.7 hours

final formula:
90miiles / 1.7 hours = 52.94 miles per hour average!

Thank-you very much!! I understand now!