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The answer is E

Given : train takes detour to reach a destination
To find: if DEtour time tn-t>2hrs

1) (S-20)<Sn<(S+20) so not sufficient to know tn-t
2) (d-40)<dn<(d+40) so not sufficient to know tn-t

Together to find the min and max, we take, (d-40)/(s+20) <tn<(d+40)/(S-20) ...We cant solve this either. So answer is E.

We can try tn-t where t=d/s...we will still end up with a unsolvable numerator and denominator
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Statement (1)
The speeds differ by no more than 20 km/hr.
This gives only information about speeds, not distances.
Even with small speed differences, the travel time difference may be small or very large, depending on distances
Not sufficient.

Statement (2)
The distances differ by no more than 40 km.
This gives only information about distances, not speeds.
Even a small distance increase could add more than 2 hours if the speed is low.
Not sufficient.

(1) + (2)

Regular:
  • 40 km at 40 km/hr => 1 hr
If Detour:
  • 80 km at 20 km/hr => 4 hr
Difference = 3 hr (>2hr)

If Detour:
  • 60 km at 50 km/hr => 6/5 hr
Difference = 1/5 hr (<2hr)

Answer: E

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A regional express train normally travels from Northgate Station to Lakeside Station by its regular route and has a fixed scheduled travel time for that route. On one day, because of track maintenance, the train traveled from Northgate to Lakeside by a detour route instead. Was the train’s travel time that day more than 2 hours longer than the scheduled travel time?

(1) The train’s average (arithmetic mean) speed on the detour route differed from its average (arithmetic mean) speed on the regular route by no more than 20 kilometers per hour.
(2) The distance the train traveled on the detour route differed from the distance on the regular route by no more than 40 kilometers.

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Hi Bunuel, (or anyone else that can answer) - I got this right but only because the word 'differed' I thought does not indicate an automatically slower relationship. as in, what if the detour was 40km shorter? is the avg speed greater or less by 20km? Should I always assume differed should be less than? it feels like a word describing magnitude, not a comparison.

Ex. My grades differed by 20 points than my peers [Is it greater by 20, or less by 20?]
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GMAT Club Official Solution:

A regional express train normally travels from Northgate Station to Lakeside Station by its regular route and has a fixed scheduled travel time for that route. On one day, because of track maintenance, the train traveled from Northgate to Lakeside by a detour route instead. Was the train’s travel time that day more than 2 hours longer than the scheduled travel time?

Let R be the distance, in kilometers, on the regular route, and let S be the train’s average speed, in kilometers per hour, on the regular route. So the scheduled travel time is:

R/S

Let D be the distance, in kilometers, on the detour route, and let V be the train’s average speed, in kilometers per hour, on the detour route. So the travel time on the detour route is:

D/V

The question asks whether:

D/V > R/S + 2

(1) The train’s average (arithmetic mean) speed on the detour route differed from its average (arithmetic mean) speed on the regular route by no more than 20 kilometers per hour.

This gives only a limit on the difference between S and V. It gives no information about R or D, so we cannot determine whether the detour travel time was more than 2 hours longer than the scheduled travel time.

Not sufficient.

(2) The distance the train traveled on the detour route differed from the distance on the regular route by no more than 40 kilometers.

This gives only a limit on the difference between R and D. It gives no information about S or V, so we cannot determine whether the detour travel time was more than 2 hours longer than the scheduled travel time.

Not sufficient.

(1)+(2) Even together, the statements are not sufficient.

For example, suppose:

R = 200
S = 100
D = 240
V = 100

Then the scheduled travel time is:

200/100 = 2 hours

The detour travel time is:

240/100 = 2.4 hours

The detour distance differs from the regular distance by 40 kilometers, and the two average speeds differ by 0 kilometers per hour, so both statements are satisfied. But the detour travel time is not more than 2 hours longer than the scheduled travel time.

So the answer is NO.

Now suppose:

R = 40
S = 20
D = 80
V = 10

Then the scheduled travel time is:

40/20 = 2 hours

The detour travel time is:

80/10 = 8 hours

The detour distance differs from the regular distance by 40 kilometers, and the two average speeds differ by 10 kilometers per hour, so both statements are satisfied. But now the detour travel time is more than 2 hours longer than the scheduled travel time.

So the answer is YES.

Not sufficient.

Answer: E.
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yogeshr2
Hi Bunuel, (or anyone else that can answer) - I got this right but only because the word 'differed' I thought does not indicate an automatically slower relationship. as in, what if the detour was 40km shorter? is the avg speed greater or less by 20km? Should I always assume differed should be less than? it feels like a word describing magnitude, not a comparison.

Ex. My grades differed by 20 points than my peers [Is it greater by 20, or less by 20?]


No. “Differed by 20” only tells you the size of the difference, not the direction.

So if the regular speed is 100, the detour speed could be 80 or 120. Both differ from 100 by 20.
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