1. Starting at noon. John drives north along a highway at a speed of 70 mi/hr until 2:30pm.
So at 2:30pm he has traveled 70 mi/hr for 2.5 hours.
How far has he traveled since noon?
Since distance = speed×time, at 2:30pm, he has traveled 70×2.5 = 175 miles.
2. He then continues in the same direction at a different steady speed for a distance of 75 miles until 4:00 pm.
So he goes 75 miles from 2:30 to 4:00pm, which is 1.5 hours.
What is his speed between 2:30 pm and 4:00pm?
since speed = distance/time, his speed is 75/1.5 = 50 mi/hr
3. From 4:00 pm he travels an additional 180 miles in the same direction to arrive
at his destination. He travels this final part of his journey at a constant speed of 60 mi/hr.
So he travels 180 miles at 60 mi/hr,
When does he get to his destination?
Since time = distance/speed, 180/60 = 3 hours after 4:00pm or 7:00pm
4. What is John's average speed between his start at noon and the time he arrives at his destination?
In part 1, he drives 175 miles.
In part 2, he drives 75 miles.
In part 3, he drives 180 miles.
So he drives a total of 175+75+180 or 430 miles
From noon to 7:00 is 7 hours.
Average speed = total distance/total time = 430/7 = 61 3/7 mi/hr.
Edwin
