Gill drives 120 miles from Los Angeles to San Diego...

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AAPL: Moderator; Posts: 2599; Joined: Sun Oct 29, 2017 2:08 pm; Followed by:2 members

Gill drives 120 miles from Los Angeles to San Diego...

by AAPL » Sat Mar 10, 2018 6:09 am

Gill drives 120 miles from Los Angeles to San Diego to fetch a package. On her way there she drives at 40 miles per hour. On her way back she drives 50% faster. What is Gill's average velocity for the round trip?

A. 24 miles per hour
B. 48 miles per hour
C. 50 miles per hour
D. 53 1/3 miles per hour
E. 68 miles per hour

The OA is B.

We know that the average from LA to SD = 40 mph.

If on her way back she drives 50% faster, then the average speed from SD to LA = (1.5)*(40) = 60 mph.

Total time will be = (120/40) + (120/60) = 3 + 2 = 5 hours.

Finally, the average speed for the round trip = 240/5 = 48 miles per hour. Option B.

Is there a strategic approach to this question? Can any experts help, please? Thanks!

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Source: Beat The GMAT — Problem Solving | Sat Mar 10, 2018 6:09 am

mbawisdom: Senior | Next Rank: 100 Posts; Posts: 94; Joined: Tue Dec 16, 2014 9:50 am; Location: London, UK; Thanked: 2 times; Followed by:4 members; GMAT Score:770

by mbawisdom » Sat Mar 10, 2018 6:14 am

AAPL wrote:Gill drives 120 miles from Los Angeles to San Diego to fetch a package. On her way there she drives at 40 miles per hour. On her way back she drives 50% faster. What is Gill's average velocity for the round trip?

A. 24 miles per hour
B. 48 miles per hour
C. 50 miles per hour
D. 53 1/3 miles per hour
E. 68 miles per hour

The OA is B.

We know that the average from LA to SD = 40 mph.

If on her way back she drives 50% faster, then the average speed from SD to LA = (1.5)*(40) = 60 mph.

Total time will be = (120/40) + (120/60) = 3 + 2 = 5 hours.

Finally, the average speed for the round trip = 240/5 = 48 miles per hour. Option B.

Is there a strategic approach to this question? Can any experts help, please? Thanks!

Hi AAPL, I would approach it in the same way you did. Divide the trips up into two parts and then solve for the total trip.

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by [email protected] » Sat Mar 10, 2018 4:29 pm

Hi AAPL,

We're told that Gill drives 120 miles from Los Angeles to San Diego to fetch a package; on her way there she drives at 40 miles per hour and on her way back she drives 50% faster. We're asked for Gill's average velocity for the round trip. This question can be approached in a number of different ways. Here's how you can solve it with almost no math at all - just a little logic (and using the 'spread' of the Answer choices to your advantage):

Gill drove 40 miles/hour in one direction and then drove back (the same distance) at a speed that was 50% FASTER (40 mph + 20 mph = 60 miles/hour). Driving the same distance at a faster speed takes LESS time, so Gill spent MORE time traveling 40 miles/hour than she spent driving 60 miles/hour. This is ultimately a 'weighted average' - meaning that the average speed for the entire trip will be CLOSER to 40 than it is to 60. Looking at the Answers, we can immediately eliminate Answer C, D and E. Since the average speed has to be BETWEEN 40 and 60, we can also eliminate Answer A (it's clearly too small). There's only one answer left...

Final Answer: B

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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by Scott@TargetTestPrep » Sun Jun 09, 2019 6:01 pm

AAPL wrote:Gill drives 120 miles from Los Angeles to San Diego to fetch a package. On her way there she drives at 40 miles per hour. On her way back she drives 50% faster. What is Gill's average velocity for the round trip?

A. 24 miles per hour
B. 48 miles per hour
C. 50 miles per hour
D. 53 1/3 miles per hour
E. 68 miles per hour

We can use the formula: average rate = total distance/total time.

The rate on the first leg is 40, and the rate on the second leg is 1.5 x 40 = 60.

The time on the first leg is 120/40 = 3, and the time on the second leg is 120/60 = 2.

The average speed is:

240/5 = 48

Answer: B

Scott Woodbury-Stewart
Founder and CEO
[email protected]