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Speed

by crackgmat007 » Wed May 13, 2009 9:20 pm
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?
A. 71.5
B. 72
C. 72.5
D. 73
E. 73.5

Can anyone help me solve this problem to get 72? tx.
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by cramya » Wed May 13, 2009 9:49 pm
Convert the seconds to hour since we are asked for km/hr and one of the speed's is given in km/hr

2 sec = 2/3600 = 1/1800 hr (since 3600 sec = 1 hr)

So one speed is given therefore time(t1) = distance/speed = 1km/75km/hr = 1/75 hr

Let x be the speed in km/hr therefore time (t2) = 1km/x km/hr = 1/x hr

t2 + 1/1800 = t1 or t2 - t1 = 1/1800

1/x-1/75 = 1/1800(i.e our 2 seconds)

x=72
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by logitech » Thu May 14, 2009 9:41 am
cramya wrote:Convert the seconds to hour since we are asked for km/hr and one of the speed's is given in km/hr

2 sec = 2/3600 = 1/1800 hr (since 3600 sec = 1 hr)

So one speed is given therefore time(t1) = distance/speed = 1km/75km/hr = 1/75 hr

Let x be the speed in km/hr therefore time (t2) = 1km/x km/hr = 1/x hr

t2 + 1/1800 = t1 or t2 - t1 = 1/1800

1/x-1/75 = 1/1800(i.e our 2 seconds)

x=72
CRAMYA WHERE ARE YOU ? :)

whenever you see seconds and the question is dealing with HOURS, think about SEC/3600 as cramya explained

2/3600 = 1/1800

Then you can use the Cramya's equation but it does not hurt to keep in mind that 1800 is divisible with 72 and 75! ( I am being a little dorky here but, this can save you some time)
LGTCH
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