Final answer:
Lisa's average speed for the entire journey is found by adding the distances she covered at different speeds and then dividing by the total travel time, resulting in an average speed of 78.4 km/h.
Step-by-step explanation:
To calculate Lisa's average speed for the entire journey, we need to determine the total distance she traveled and divide it by the total time of the journey.
First, she drove at 76 km/h for 30 minutes, which is 0.5 hours. The distance covered during this time is:
Distance = Speed × Time
= 76 km/h × 0.5 h
= 38 km
Then, she drove 80 km/h for 45 minutes, which is 0.75 hours. The distance for this part of the journey is:
Distance = Speed × Time
= 80 km/h × 0.75 h
= 60 km
Adding both distances gives us a total distance of:
Total Distance = 38 km + 60 km
= 98 km
The total journey time is 30 minutes + 45 minutes = 75 minutes or 1.25 hours. So, her average speed is:
Average Speed = Total Distance / Total Time
= 98 km / 1.25 h
Average Speed = 78.4 km/h