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!
Gill drives 120 miles from Los Angeles to San Diego...
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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.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!
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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
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
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We can use the formula: average rate = total distance/total time.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 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
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