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by rupsk » Sun Aug 07, 2011 6:32 pm
two cars enter a 6 mile tunnel from each end at the same time. how many seconds after they enter do they meet?
1 the sum of their speed is 120m/hr
2. they meet at the point 4 miles from one end of the tunnel.

Need an explanation for (1) as I know (2) alone is not sufficient.
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Source: — Data Sufficiency |
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by sunnyjohn » Sun Aug 07, 2011 10:03 pm
Concept of relative speed.

As trains are coming from opposite sides, so total time would be = (Total distance)/(Total speed)
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by ColumbiaVC » Sun Aug 07, 2011 10:30 pm
d=6 miles
so, d=d1+d2 (d1=r1t1 and d2=r2t2)
=> d=r1t1+r2t2

Same time traveled by both the cars so t1=t2=t

d=t(r1+r2)
d=6 miles
1) r1+r2=120 m/hr
t=d/(r1+r2)
Substitute the values and calculate the time. Hence [spoiler]Sufficient![/spoiler]
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by gmatboost » Sun Aug 07, 2011 11:36 pm
When two things are moving right at one another, the sum of their speeds tells us the rate at which they are getting closer to each other.

Whether one is going 30 mi/hr and the other 90 mi/hr, or 80 and 40, or 60 and 60, the gap between them closes at the combined rate of 120 mi/hr.

So, if we know the distance and the rate, [spoiler]we can use R*T = D to find the time.[/spoiler]
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