QR Question

[doc1986]

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This seems like an extremely easy question but for some reason I don't understand where they are getting their answer.

A train traveled from Town A to Town B at an average speed of 48 miles per hour. The train returned from Town B to Town A along the same route at an average speed of 96 miles per hour. What was the average speed of the train, in miles per hour, for the round trip?
A. 64
B. 68
C. 72
D. 80
E. 84

I did (48 + 96)/2 = 72.

Why is this not the correct answer?

Thanks

 

[razblo]

Not sure if this is right or not but I got A (64).

Your logic is off because you're assuming that the train takes as long to get from point A to point B as it does to go from point B to point A when the speeds are different.

Let's say the distance from point A to point B is 96 miles.

It would take 2 hours for the train to get from point A to point B at 48 mph and one hour to get back at 96 mph.

So you would have to take into consideration that you're traveling at 48 mph for two hours compared to traveling at 96 mph for one hour.

This is how I set it up.

48 mph x 2 hours = 96 miles. 96 mph x 1 hour = 96 miles.

96 miles + 96 miles = 192 miles.

192 miles / 3 hours = 64 mph.

 

[purplepanda]

awesome job razblo. I'm going to write this one in my notes, bc...I didn't see that right away. In my head I was thinking...why didn't they give distance so that I could find the total d/total t!

 

[doc1986]

Thanks!

 

[hsia2]

i am just wondering if anyone uses any other resources to study QR aside from CDM, Topscore and destroyer?
After hearing so many horror stories, I am just not sure those 3 are enough
(people often score like 16/ 17 or even 14 just using those)...

 

[razblo]

i am just wondering if anyone uses any other resources to study QR aside from CDM, Topscore and destroyer?
After hearing so many horror stories, I am just not sure those 3 are enough
(people often score like 16/ 17 or even 14 just using those)...

Some of it probably has to do with some people being naturally better at math than others but I would guess the biggest part is most people don't study math as much as the science subjects.

 

[PooyaH]

For these type of question you need to know the distance formula: Distance = Speed x Time
when you have 2 different speeds given, always find the total time (Time = Distance / Speed) and divide it by the total distance to get the avg speed (Speed = Distance / Time)