Another Rate Problem

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DCJ: Senior | Next Rank: 100 Posts; Posts: 68; Joined: Mon May 04, 2009 10:01 pm

by DCJ » Thu Jul 23, 2009 10:48 pm

Susan drove an average speed of 30mph for the first 30 minutes of a trip and then at an average speed of 60mph for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed in mph for the whole trip?

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Source: Beat The GMAT — Problem Solving | Thu Jul 23, 2009 10:48 pm

sreak1089: Master | Next Rank: 500 Posts; Posts: 379; Joined: Wed Jun 03, 2009 3:05 amFollowed by: 1 member; Thanked: 19 times; GMAT Score:690

by sreak1089 » Fri Jul 24, 2009 3:05 am

45 miles per hour is the answer.

distance travelled first 1 hr = (1/2 hr * 30) = 15 miles (in 1/2 hr)
time taken for 30 miles @ 60 miles/hr = (30/6) = 1/2 hr

Total time taken = 1/2 + 1/2 = 1 hour
Total distance travelled = 15 + 30 = 45 miles
Avg speed = 45 miles/hr

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truplayer256: Master | Next Rank: 500 Posts; Posts: 392; Joined: Thu Jan 15, 2009 12:52 pm; Location: New Jersey; Thanked: 76 times

by truplayer256 » Fri Jul 24, 2009 4:36 am

If my memory serves me correctly, I think the problem is supposed to say:"Susan drove an average speed of 30mph for the first 30 miles of a trip."

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sreak1089: Master | Next Rank: 500 Posts; Posts: 379; Joined: Wed Jun 03, 2009 3:05 amFollowed by: 1 member; Thanked: 19 times; GMAT Score:690

by sreak1089 » Fri Jul 24, 2009 4:44 am

I understood this way. Susan drove x miles for first 30 minutes @ 30 miles/hr. ==> x = (distance x time) = (1/2 hr x 30 miles/hr) = 15 miles.

==> 15 miles in 1/2 hour.

Later, drove further 30 miles @ 60 miles per hour.
==> time = (distance / speed) = (30/60) = 1/2 hour.

So she travelled 15 miles in first 1/2 hour and further 30 miles in the next 1/2 hour.

So she travelled 45 miles in 1 hour. Hence avg speed = 45 miles / hour.

truplayer256 wrote:If my memory serves me correctly, I think the problem is supposed to say:"Susan drove an average speed of 30mph for the first 30 miles of a trip."

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DCJ: Senior | Next Rank: 100 Posts; Posts: 68; Joined: Mon May 04, 2009 10:01 pm

by DCJ » Fri Jul 24, 2009 10:13 am

I keep getting 45mph as well using the same process but for some reason the answer in 40mph. Can anyone shine some light on that? Otherwise since this is a Kaplan problem it is possible that they made a mistake (as they are known to do).

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jjk: Senior | Next Rank: 100 Posts; Posts: 43; Joined: Thu Mar 12, 2009 8:56 pmFollowed by: 1 member; Thanked: 10 times; GMAT Score:730

by jjk » Fri Jul 24, 2009 11:42 am

truplayer256 is right; you read the problem incorrectly.

It's supposed to say that Susan drove at an average speed of 30 mph in the first 30 miles of the trip. That's why you're getting the wrong answer of 45 mph.

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