M40-120

Question

During its supply delivery trip, did Drone Z ever exceed a speed of 120 kilometers per hour?

(1) Drone Z took more than 4 hours to complete the trip.
 
(2) Drone Z traveled a total of 480 kilometers during the trip.

Bunuel · Answer

Official Solution:

During its supply delivery trip, did Drone Z ever exceed a speed of 120 kilometers per hour? 
 
(1) Drone Z took more than 4 hours to complete the trip.
 
Clearly insufficient.
 
(2) Drone Z traveled a total of 480 kilometers during the trip.
 
Clearly insufficient.
 
(1)+(2) If the trip covered 480 km in more than 4 hours, then the average speed was below 120 km/h. But that still doesn’t answer whether Drone Z ever exceeded 120 km/h at any point. Even with an average below 120, it’s possible that the speed exceeded 120 for part of the trip. Not sufficient.
 
Answer: E

BinodBhai · Answer

I don’t quite agree with the solution. doesn't make sense

Bunuel · Answer

If you study the official solution carefully enough, it will actually make perfect sense. The key is that knowing the average speed is below 120 km/h does not rule out the possibility that the drone exceeded 120 km/h at some point. For example, the drone could have flown faster than 120 for part of the trip and then much slower at other times, resulting in an overall average below 120. Alternatively, it could have traveled at a constant speed of exactly 120 km/h. So neither statement alone nor both together can prove whether Drone Z ever went above 120 km/h. That’s why the answer is E. If still not clear you can check alternative solution here: https://gmatclub.com/forum/during-its-s ... l#p3566985 P.S. Saying “doesn’t make sense” is not a valid criticism. If there is something specific you don’t understand, please ask a specific question so it can be addressed clearly.

BinodBhai · Answer

Now it makes sense to me.. earlier I couldn't understand it. Thanks, Bunuel.

.

utkarsh2025v2 · Answer

I don’t quite agree with the solution. I combine both these

1. Time Range: 4h 1min to 480h to infinity

2. Distance = 480

Speed = D/T = 480/(4h 1min) - 480/480h = 119 - 1

The value never exceeded more 120, Please fix solution

Bunuel · Answer

There is nothing to fix. The question is absolutely correct, so please review it and study more carefully.

Your post confuses average speed with instantaneous speed. Showing that the  average  stays below 120 does not prove the drone never went above 120 at some moment. A vehicle can average 100 and still hit 150 for a short stretch. That is why your argument doesn’t work and why the answer remains E.

vanshitabajaj26 · Answer

But how do we get to know that we have to check for the avg speed?

Bunuel · Answer

Because statements (1) and (2) only give you  time  and  distance .

The only thing you can extract from time and distance is the  average  speed, not the maximum speed, which is exactly why the info is insufficient.

Review the solution and the discussion more carefully.

M40-120

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