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Bunuel
Al and Barb shared the driving on a certain trip. What fraction of the total distance did Al drive?

(1) Al drove for 3/4 as much time as Barb did. We don't know their average speeds. Not sufficient.

(2) Al's average driving speed for the entire trip was 4/5 of Barb's average driving speed for the trip. We don;t know the times. Not sufficient.

(1)+(2) When combined we have all we need:

Barb's time = t hours, Al's time = 3/4*t hours.
Barb's speed = r miles per hour and Al's rate = 4/5*r miles per hour.

Barb's distance = rt and Al's distance = 3/5*rt --> Al's covered (3/5*rt)/(rt+3/5*rt)=3/8 of the total distance. Sufficient.

Answer: C.

Hi Bunuel
Great explanation.
I had a doubt that since we know that time is proportional to distance if the time taken by Al is 3/4 of the time of B, can't we infer from here that the ratio of there distances is 3/4 and Al drove 3/7 of the total distance??????

Thanks in advance
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Bunuel
Al and Barb shared the driving on a certain trip. What fraction of the total distance did Al drive?

(1) Al drove for 3/4 as much time as Barb did. We don't know their average speeds. Not sufficient.

(2) Al's average driving speed for the entire trip was 4/5 of Barb's average driving speed for the trip. We don;t know the times. Not sufficient.

(1)+(2) When combined we have all we need:

Barb's time = t hours, Al's time = 3/4*t hours.
Barb's speed = r miles per hour and Al's rate = 4/5*r miles per hour.

Barb's distance = rt and Al's distance = 3/5*rt --> Al's covered (3/5*rt)/(rt+3/5*rt)=3/8 of the total distance. Sufficient.

Answer: C.

Hi Bunuel
Great explanation.
I had a doubt that since we know that time is proportional to distance if the time taken by Al is 3/4 of the time of B, can't we infer from here that the ratio of there distances is 3/4 and Al drove 3/7 of the total distance??????

Thanks in advance

No. Say you drive twice as fast as I do. Can we say that you cover twice as much distance as I do? No, because we don't how long each of us drives. If you drive for 1 hour and I drive for 100 hours, what fraction of the total distance drive?
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Bunuel
Al and Barb shared the driving on a certain trip. What fraction of the total distance did Al drive?

(1) Al drove for 3/4 as much time as Barb did. We don't know their average speeds. Not sufficient.

(2) Al's average driving speed for the entire trip was 4/5 of Barb's average driving speed for the trip. We don;t know the times. Not sufficient.

(1)+(2) When combined we have all we need:

Barb's time = t hours, Al's time = 3/4*t hours.
Barb's speed = r miles per hour and Al's rate = 4/5*r miles per hour.

Barb's distance = rt and Al's distance = 3/5*rt --> Al's covered (3/5*rt)/(rt+3/5*rt)=3/8 of the total distance. Sufficient.

Answer: C.
——-
Thanks bunuel for this explanation.

Unfortunately, it took me a long time to arrive at this equation. I also took a value for distance and tried to find the ans - I realised later that I couldn’t do that because we’re trying to find the fraction of the distance itself.

Pls correct me if my logic is incorrect

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total distance = average speed * total time

(1) We don't know the average speed of Al or Barb -- we only know the times. Al could have drove 1000 miles and Barb could have drove 2 miles and vice versa. Insufficient.

(2) We don't know the time spent driving by Al or Barb. Insufficient.

(1 & 2) Combined we can determine the total distance.

Barb's distance = rt
Al's distance = 3/4t * 4/5r = 3/5rt

Total distance = rt + 3/5rt
Al covered: (3/5rt) / (rt + 3/5rt) = 3/8 of the trip
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Hi Bunuel,

Trying to seek clarification regarding something slightly related. I was made to understand that 'average' speed would be insufficient in answering rate problems. The question stem has to specify that the speed is 'constant'.

So, is there a way to know for sure that a problem can be solved using either case?

cc: KarishmaB

Thank you
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Vaishbab
Hi Bunuel,

Trying to seek clarification regarding something slightly related. I was made to understand that 'average' speed would be insufficient in answering rate problems. The question stem has to specify that the speed is 'constant'.

So, is there a way to know for sure that a problem can be solved using either case?

cc: KarishmaB

Thank you
Vaishbab Understanding Average vs. Constant Speed in GMAT Problems

You're right to be careful about this distinction, but let me clarify when each matters:

Key Insight: The need for "constant" speed depends on what the question is asking, not just whether average speed is mentioned.

When Average Speed IS Sufficient:
- When calculating \(\text{total distance} = \text{average speed} \times \text{total time}\)
- When comparing overall distances traveled (like in this problem)
- When the question asks about the entire trip as a whole

When Constant Speed IS Needed:
- When you need to know the exact position at a specific moment
- When calculating meeting points or passing times
- When the problem involves changes in speed during the journey

Why This Problem Works with Average Speed:
In this DS question, we only need the total distances each person drove. Since:
\(\text{Total Distance} = \text{Average Speed} \times \text{Total Time}\)

The average speeds given in Statement 2 are perfectly adequate for calculating the fraction of total distance.

Decision Framework - First 5 Seconds:
1. Does the question ask about total/overall quantities? → Average speed works
2. Does the question ask about specific moments/positions? → Need constant speed
3. Are there multiple segments with different speeds? → Check what's being asked

Common GMAT Pattern:
You'll see this same logic in:
- Work rate problems (average rate vs. varying rates)
- Investment return problems (average return vs. yearly returns)
- Production rate problems (average output vs. hourly output)
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Vaishbab
Hi Bunuel,

Trying to seek clarification regarding something slightly related. I was made to understand that 'average' speed would be insufficient in answering rate problems. The question stem has to specify that the speed is 'constant'.

So, is there a way to know for sure that a problem can be solved using either case?

cc: KarishmaB

Thank you

Average speed over a distance can be assumed to be constant over that distance if the distance is not split into parts. So for most of our calculations, if it is known that A drove at an average speed of 50 mph for the first 100 miles, then we can assume that A drove at this speed for the entire 100 mile distance. But if this 100 miles distance is split into parts of say 80 miles and 20 miles and we are asked the average speed in the first 80 miles, then we cannot say what it will be because we only have average speed over the entire 100 mile distance given to us. The average speed over 80 miles could be very different from 50 mph. In that case, we need to be given the constant speed of the 100 mile journey to be able to say what the speed will be over 80 miles.
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