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Question

You travel on the highway at a rate of 60mph for 1 hour and at 50mph for 2 hours and 40mph for 3 hours. What is your average speed during the trip?

A
48mph
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B
47mph
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C
46mph
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D
45mph
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Solution

The correct option is C 47mph
Divide the whole trip into three intervals;
1. Travel at a speed of 60mph for 1hr.
2. Travel at a speed of 50mph for 2hr.
3. Travel at a speed of 40mph for 3hr.
x=vt
1. Speed is 60mph, time is 1hr, then
x1=v1t1=60×1=60miles
2. Speed is 50mph, time is 2hr, then
x2=v2t2=50×2=100miles
3. Speed is 40mph, time is 3hr, then
x3=v3t3=40×3=120miles
Then the total traveled distance is
xtotal=x1+x2+x3
=60+100+120=280miles
The average speed is defined as the ratio of total traveled distance and the total traveled time. The total traveled time is
ttotal=t1+t2+t3=1+2+3=6hr
Average speed
vav=Xtotalttotal=2806=40×3
=47mph

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