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Why does time matter

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by MalcolmW » Tue Nov 17, 2015 6:16 pm

Timer

00:00

Answers

A

B

C

D

E

Stats

Difficulty—

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.

I chose A as I normally practice not having to go into depth with solving problems to save time. I thought statement 2 gave me information that wasn't needed. Can someone explain the answer?
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Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Tue Nov 17, 2015 8:30 pm
MalcolmW wrote: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.
Target question: What was Mary's average speed for the 50-mile trip?

Average speed = (total distance)/(total travel time)
= (50 miles)/(total travel time)

As you can see, we need only determine the total travel time.

Statement 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.
For both parts of her journey, we COULD determine the travel time, so we COULD find the total travel time, which means we COULD calculate Mary's average speed
Since we COULD answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: She drove a total of 54 minutes.
Perfect! Her total travel time is 54 minutes, so we COULD calculate Mary's average speed
Since we COULD answer the target question with certainty, statement 2 is SUFFICIENT

Answer = D

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Max@Math Revolution » Sat Nov 21, 2015 9:09 am
Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

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.

If we modify the original condition, vt=50(v:velocity, t:time taken), there are 2 variables (v,t) and one equations (yt=50) and 2 equations, giving a high chance (D) will be our answer.
Condition 1) t=54 (30min+24min)
Condition 2) t=54min
Each are sufficient, making the answer (D).

For cases where we need 1 more equation, such as original conditions with "1 variable", or "2 variables and 1 equation", or "3 variables and 2 equations", we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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by MalcolmW » Sat Nov 21, 2015 11:49 pm
ill be taking my test on 12/12 i just heard of this method. I'm not sure if that is enough time to implement it into my practice/studying.
Max@Math Revolution wrote:Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

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.

If we modify the original condition, vt=50(v:velocity, t:time taken), there are 2 variables (v,t) and one equations (yt=50) and 2 equations, giving a high chance (D) will be our answer.
Condition 1) t=54 (30min+24min)
Condition 2) t=54min
Each are sufficient, making the answer (D).

For cases where we need 1 more equation, such as original conditions with "1 variable", or "2 variables and 1 equation", or "3 variables and 2 equations", we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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by [email protected] » Mon Mar 26, 2018 3:41 pm
Hi All,

On a recent trip, Mary drove 50 miles. What was the AVERAGE SPEED at which she drove the 50 miles. A few of the DS questions that you'll face on Test Day are essentially 'logic' questions - meaning that you can answer them without actually doing any math (as long as you understand the concept(s) involved). Here, we're given the TOTAL DISTANCE and asked to find the AVERAGE SPEED. If we have the TOTAL TIME, then we can definitively answer this question.

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.

With the information in Fact 1, we can determine the total time for each 'leg' of the trip, so we'll have the total time for the entire trip and we can answer the given question.
Fact 1 is SUFFICIENT

2) She drove a total of 54 minutes.

With the information in Fact 2, we're given the total time, so we can answer the given question.
Fact 2 is SUFFICIENT

Final Answer: D

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by Jeff@TargetTestPrep » Thu Jul 26, 2018 3:29 pm
MalcolmW wrote: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.
We are being tested on average speed. The formula for average speed is:

average speed = total distance/total time

We are given that the total distance is 50 miles, so we can substitute 50 miles into our formula.

average speed = 50/total time

Thus, if we can determine the total time, we can determine the average speed.

Statement One Alone:

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.

We are given that Mary first drove 30 miles at an average speed of 60 mph.

Since time = distance/rate, we can say:

time for the first 30 miles = 30/60 = ½ hour

We are also given that Mary drove the remaining 20 miles at an average speed of 50 miles per hour. Thus, we can say:

time for the remaining 20 miles = 20/50 = 2/5 hours

So we know that total time = ½ + 2/5 = 5/10 + 4/10 = 9/10 hours = 54 minutes. Since we have the total time, we can determine average speed. Statement one is sufficient to answer question.

Statement Two Alone:

She drove a total of 54 minutes.

We are given the total time, so statement two is also sufficient to answer the question.

Answer: D

Jeffrey Miller
Head of GMAT Instruction
[email protected]

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