Average speed is 18.6 kilometer per hour
Solution:
Given that,
To visit the sites, Nathan's family drove 6.8 km, 9.8 km, 20.3 km, and 46.8 km
They drove for 4.5 hour
To find: average speed
The average speed is given by formula:

Here, total distance = 6.8 + 9.8 + 20.3 + 46.8 = 83.7 km
Total time taken = 4.5 hour
Substituting the values in formula,

Thus average speed is 18.6 kilometer per hour