siggarusfigs wrote:
hey,
why would we count the last stop for the express train? it has already traveled 600 km the instant it gets to the last stop. therefore, shouldn't the number of 3 minute breaks be 6 rather than 7?
And thus, the express train travels 600 km in (8 intervals of 75km)*(45 minutes) + (6 stops)*(3 minutes) = 378 minutes
Then, counting the last stop of the local train, because it will need to travel some additional amount to fill the remaining time:
The time it takes the local train to travel 300km: (12 intervals of 25km)*(30 minutes) + (11 stops)*(1 minute) = 371 minutes
Which leaves 7 minutes left over-- allowing the local train to travel an additional: (7 minutes/60 minutes)*(50 km/hour) = 5.83 km
Which adds up to 12*25 + 5.83 = 305.83km traveled by the local train...
At the end of 600 km, you have the 8th stop, not the 7th stop. That 8th stop has not been counted.
Every 75 km, one stop. So after 150 km, you would have had 2 stops. So after 300 km, you would have had 4 stops and hence after 600 kms, 8 stops.
Also, the question is a little ambiguous.
The train travels at an average speed of 100 km/hr does not specify whether the stops are a part of the time taken to calculate average speed or not. Better verbiage would be "a train runs at an average speed of 100 km/hr and then stops for 3 mins after 75 km" and so on...
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Karishma
Veritas Prep GMAT Instructor
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