Welcome! Check out our free B-School Guides to learn how you compare with other applicants.

## Average Speed

This topic has 1 expert reply and 3 member replies
kayser Just gettin' started!
Joined
05 Feb 2009
Posted:
9 messages
Test Date:
Yesterday
Target GMAT Score:
201
GMAT Score:
200
Average Speed Thu Feb 05, 2009 12:46 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
On a recent trip, Mary drove 50 miles. What was the average speed at which she drove the 50 miles?

(1) She drove 30 miles at an average speed of 60 miles per hour and then drove the remaining 20 miles at an average speed of 50 miles per hour.

(2) She drove a total of 54 minutes.

How would you do this problem using statement 1, if it were a Problem Solving question?

Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
Joined
01 Jan 2009
Posted:
2567 messages
Followed by:
408 members
Thanked:
596 times
GMAT Score:
770
Thu Feb 05, 2009 2:01 am
Each statement by itself is sufficient.

Take 1: Mary drove for 30/60 = half an hour with an average speed of 60 miles/h and then she drove for 20/50 = 0.4 hours with an average speed of 50 miles/h. This means she drove 50 miles in 0.9 hours, which gives us an average speed of 50/0.9 = 55.56 miles/h

Take 2: Total distance = 50 and total time = 54 mins = 0.9 h. So average speed = total dist/total time = 50/0.9 = 55.56 miles/h.

kayser Just gettin' started!
Joined
05 Feb 2009
Posted:
9 messages
Test Date:
Yesterday
Target GMAT Score:
201
GMAT Score:
200
Fri Feb 06, 2009 11:04 am
So, for the first statement - if this was a PS question - is there a shortcut?

Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

Thanks alot.

Joined
01 Jan 2009
Posted:
2567 messages
Followed by:
408 members
Thanked:
596 times
GMAT Score:
770
Fri Feb 06, 2009 11:45 am
Well, trust me, it's pretty short if you just write down the equations. I don't really know of any shorter method, but maybe someone else can help.

### GMAT/MBA Expert

Stuart Kovinsky GMAT Instructor
Joined
08 Jan 2008
Posted:
3156 messages
Followed by:
461 members
Thanked:
1433 times
GMAT Score:
800
Fri Feb 06, 2009 12:38 pm
kayser wrote:
So, for the first statement - if this was a PS question - is there a shortcut?

Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

Thanks alot.
A great way to attack multiple-part speed questions is with a chart.

Across the top of the chart, write the formula, D = r * t.
Down the left side of the chart, write the different parts of the journey, e.g.:

part 1
part 2
total

or

uphill
downhill
total

So, for this question, your chart will look something like this (excuse my poor formatting!):

.................Distance....=...........Rate.........*...........Time

Part 1...........30........................60...............................

Part 2...........20........................50...............................

Total.............50........................x................................

The chart allows you to quickly assimilate the information you're given and plan your route to solve the question.

In this case, we'd see that we can calculate the times for the individual legs, then if we add those together we get total time. Since:

Avg Rate = Total Distance/Total Time,

we'd then have everything we need to calculate the final answer.

The chart is simply a visualization of the equations to which Dana referred, but a lot of people love it because it lays out all the pieces so neatly.

_________________
Stuart Kovinsky, B.A. LL.B.
Toronto Office
1-800-KAP-TEST
www.kaptest.com

Free GMAT Practice Test under Proctored Conditions! - Find a practice test near you or live and online in Kaplan's Classroom Anywhere environment. Register today!

### Best Conversation Starters

1 varun289 38 topics
2 killerdrummer 22 topics
3 sana.noor 20 topics
4 sanaa.rizwan 14 topics
5 guerrero 14 topics
See More Top Beat The GMAT Members...

### Most Active Experts

1 Brent@GMATPrepNow

GMAT Prep Now Teacher

204 posts
2 GMATGuruNY

The Princeton Review Teacher

136 posts
3 Jim@StratusPrep

Stratus Prep

100 posts
4 Anju@Gurome

Gurome

74 posts