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susan drove at an average speed of 30 miles per hour for the

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Manager
Joined: 05 Jul 2006
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susan drove at an average speed of 30 miles per hour for the [#permalink]  25 Aug 2006, 09:02
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susan drove at an average speed of 30 miles per hour for the first 30 miles and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. if she made no stops during the trip, what was susan's average speed, in miles per hour, for the entire trip?

35
40
45
50
55
Intern
Joined: 22 Aug 2006
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40 is the correct choice:

Explantion:

(30+30) div (1+.5)
Manager
Joined: 25 May 2006
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40

1st half: (30)t1=30 --> t1=1h
2nd half: (60)t2=30 --> t2=1/2h

Total Avrg S.=120/3=40m/h
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Who is John Galt?

Senior Manager
Joined: 14 Jul 2005
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lan583 wrote:
Ruby wrote:
40 is the correct choice:

Explantion:

(30+30) div (1+.5)

should it be (1+2) instead of (1+.5)?

it should be 1 + 0.5, at 60 mph you will cover 30 miles in 1/2 an hour
Director
Joined: 28 Dec 2005
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Kudos [?]: 8 [0], given: 0

Total distance = 60
Total time = 30/30 + 30/60 = 1.5

Avg speed = d/t = 60/1.5 = 40
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