average speed formula problem
Mathematics

Tutor: None Selected  Time limit: 1 Day 
You drive for 100 miles at 65mph. then you drive for 80 miles at 75mph. What is your average speed for the trip?
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During the course of my drive, i travel in two successive speeds:
For a distance of S 1=100 miles, the speed of my car is v 2=65 mph(miles per hour)
This means I took a time of t 1=100/65 h r s. i. e. 20/13 h r s
During second course of my drive:
For a distance of S 2=80 miles, the speed of my car is v 2= 75 miles (miles per hour)
This means I took a time of t 2=80/75 h r s. i. e. 16/15 h r s
Total time taken in the journey=t 1+t 2
= 20/13+16/15
=(300+208)/195
=508/195
=2.605 h r s
Total distance travelled during the journey=s 1+s 2
=100+80 miles
=180 miles
Average speed of my trip= Total distance /Total time
=180/2.605
=69.1 mph
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