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Why does time matter

This topic has 2 expert replies and 1 member reply
MalcolmW Junior | Next Rank: 30 Posts Default Avatar
Joined
01 Nov 2015
Posted:
20 messages

Why does time matter

Post Tue Nov 17, 2015 6:16 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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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    Post 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

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    GMAT/MBA Expert

    Max@Math Revolution Legendary Member
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    Post 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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    MalcolmW Junior | Next Rank: 30 Posts Default Avatar
    Joined
    01 Nov 2015
    Posted:
    20 messages
    Post 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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