Answer:
Explanation:
Average speed is total distance divided by total time.
The relation between time, speed, and distance is ...
time = distance/speed
Total time
The time to travel 5 mi at 35 mi/h is ...
time = (5 mi)/(35 mi/h) = 1/7 h
30 seconds is (30 s)/(3600 s/h) = 1/120 h, so the total time for this trip is ...
1/7 +1/120 = (120 +7)/840 = 127/840 hours
Total distance
At 80 mph, the distance traveled in 1/120 hour is ...
distance = speed × time = (80 mi/h)(120 h) = 2/3 mi
So, the total distance traveled is ...
(5 mi) +(2/3 mi) = 17/3 mi
Average speed
The average speed is then ...
speed = distance/time
speed = (17/3 mi)/(127/840 h) = 17·840/(3·127) mi/h = 37 61/127 mi/h
The average speed is about 37.48 miles per hour.
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Additional comment
In order for the average speed to be 55.56 mph, the time spent at 80 mph would need to be about 7 minutes 12.6 seconds, for a distance at 80 mph of about 9.61 mi.