You drive for 30 min at 100 km/h and then stop for 15 min. you then drive for 45 min and 80 km/h. you average speed for the entire trip is

  So basically you drove 1/2 hour at 100 km/h which works out to a total travel distance of 50 km (you multiply a 1/2 by 100) 

Then you drove for 3/4 hour at 80 Km/h which works out to a total travel distance of 60 km (multiply 3/4 by 80) 

 now you know that you traveled a total of 110 km 

now add up all the times 30 + 15 + 45 = 90 minutes or 1.5 hours 

 now you know that you traveled a total of 110 km in 1.5 hours 

so your average speed is 73.33 km/h for the entire trip