Steve traveled the first 2 hours of his journey...

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

Steve traveled the first 2 hours of his journey...

by BTGmoderatorLU » Wed Mar 07, 2018 2:19 pm

Steve traveled the first 2 hours of his journey at 40 mph and the remaining 3 hours of his journey at 80 mph. What is his average speed for the entire journey?

A. 60 mph
B. 56.67 mph
C. 53.33 mph
D. 64 mph
E. 66.67 mph

The OA is D.

I'm confused by this PS question. Experts, any suggestion, please? Thanks in advance.

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Source: Beat The GMAT — Problem Solving | Wed Mar 07, 2018 2:19 pm

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 » Wed Mar 07, 2018 2:57 pm

LUANDATO wrote:Steve traveled the first 2 hours of his journey at 40 mph and the remaining 3 hours of his journey at 80 mph. What is his average speed for the entire journey?

A. 60 mph
B. 56.67 mph
C. 53.33 mph
D. 64 mph
E. 66.67 mph

The OA is D.

I'm confused by this PS question. Experts, any suggestion, please? Thanks in advance.

Speed = Distance/Time
OR
Distance = Speed * Time

Part 1: D1 = 40*2 = 80

Part 2: D2 = 80*3 = 240

Total Distance = D1 + D2 = 80 + 240 = 320
Total Time = 2 + 3 = 5

Speed = 320/5 = 64 mph

Answer is D

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by GMATGuruNY » Thu Mar 08, 2018 3:33 am

LUANDATO wrote:Steve traveled the first 2 hours of his journey at 40 mph and the remaining 3 hours of his journey at 80 mph. What is his average speed for the entire journey?

A. 60 mph
B. 56.67 mph
C. 53.33 mph
D. 64 mph
E. 66.67 mph

2 hours are spent at 40mph.
3 hours are spent at 80mph.
Average speed for all 5 hours = (2*40 + 3*80)/5 = 320/5 = 64.

The correct answer is D.

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DavidG@VeritasPrep: Legendary Member; Posts: 2663; Joined: Wed Jan 14, 2015 8:25 am; Location: Boston, MA; Thanked: 1153 times; Followed by:128 members; GMAT Score:770

by DavidG@VeritasPrep » Thu Mar 08, 2018 12:12 pm

LUANDATO wrote:Steve traveled the first 2 hours of his journey at 40 mph and the remaining 3 hours of his journey at 80 mph. What is his average speed for the entire journey?

A. 60 mph
B. 56.67 mph
C. 53.33 mph
D. 64 mph
E. 66.67 mph

The OA is D.

I'm confused by this PS question. Experts, any suggestion, please? Thanks in advance.

Mitch's and Dan's approach is probably simplest. But here's a fun alternative:

If Steve had spent the same amount of time traveling at each speed, his average would have been right in the middle of 40 and 80, or 60mph. We know he spent more time traveling at the faster speed, and thus, his average must be more than 60. That leaves us D and E.

Now we can test one of the two remaining choices and see if we get a 3:2 ratio in favor of time spent traveling at 80mph.
Test D: 64
Plot the rates on a number line:
40-----------------64-----------80
Gaps:-------24----------16--------
Ratio of time spent at 80 mph to 40 mph: 24:16 = 3:2. And we've got our answer.

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Thu Jun 06, 2019 5:06 pm

BTGmoderatorLU wrote:Steve traveled the first 2 hours of his journey at 40 mph and the remaining 3 hours of his journey at 80 mph. What is his average speed for the entire journey?

A. 60 mph
B. 56.67 mph
C. 53.33 mph
D. 64 mph
E. 66.67 mph

Steve's total distance was 2 x 40 + 3 x 80 = 80 + 240 = 320 miles.

We can now determine average speed:

Average speed = (total distance)/(total time)

Average speed = 320/5 = 64 mph

Answer: D

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