Distance question

Question

A family is son a road trip from los angeles to San Fransisco. For the first 20 miles they averaged 40mph. For the next 300 miles, they averaged 75mph,. If they drive 80mph for the next hour, what would their average speed be for the entire trip? 

Answer 1

20 miles x 1 hour/40 miles = 0.5 hours

300 miles x 1 hours/75 hours = 4 hours 

80 miles/hr x 1 hour = 80 miles

Total miles = 20 + 300 + 80 = 400 miles

Total hours = 0.5 + 4 + 1 = 5.5

400 miles/5.5 hours = 73 mph