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 ProblemsYou'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)