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A car traveling at a certain constant speed takes 2 seconds [#permalink]

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28 Jan 2012, 04:10

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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 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

Many approaches are possible, one of them:

Let the distance be 1 kilometer.

Time to cover this distance at 75 kilometers per hour is 1/75 hours = 3,600/75 seconds = 48 seconds; Time to cover this distance at regular speed is 48+2=50 seconds = 50/3,600 hours = 1/72 hours; So, we get that to cover 1 kilometer 1/72 hours is needed --> regular speed 72 kilometers per hour (rate is a reciprocal of time or rate=distance/time).

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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27 Nov 2014, 14:50

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Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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27 Jan 2015, 03:24

Bunuel wrote:

RadhaKrishnan wrote:

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

Many approaches are possible, one of them:

Let the distance be 1 kilometer.

Time to cover this distance at 75 kilometers per hour is 1/75 hours = 3,600/75 seconds = 48 seconds; Time to cover this distance at regular speed is 48+2=50 seconds = 50/3,600 hours = 1/72 hours; So, we get that to cover 1 kilometer 1/72 hours is needed --> regular speed 72 kilometers per hour (rate is a reciprocal of time or rate=distance/time).

Answer: B.

Dear BUnuel,

Do you confirm this approach?

75 km is 75000 m and 1 hour is 60 min. so we have : 1000*60 /75000 = 4/5 min OR 4/5 *60 = 48 seconds AS the promblem says take 2 seconds longer so we have 48+2 =50 seconds to cover 1 km or 1000 m as we have formula like this: V*t = d so we have v*50= 1000 Or v= 1000/50 = 20 m/s

NOW convert to Km/h : 20 * 3600 (s) /1000 m = 72000/1000 = 72 km/h

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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04 Feb 2015, 06:02

mehrdadtaheri92 wrote:

Bunuel wrote:

RadhaKrishnan wrote:

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

Many approaches are possible, one of them:

Let the distance be 1 kilometer.

Time to cover this distance at 75 kilometers per hour is 1/75 hours = 3,600/75 seconds = 48 seconds; Time to cover this distance at regular speed is 48+2=50 seconds = 50/3,600 hours = 1/72 hours; So, we get that to cover 1 kilometer 1/72 hours is needed --> regular speed 72 kilometers per hour (rate is a reciprocal of time or rate=distance/time).

Answer: B.

Dear BUnuel,

Do you confirm this approach?

75 km is 75000 m and 1 hour is 60 min. so we have : 1000*60 /75000 = 4/5 min OR 4/5 *60 = 48 seconds AS the promblem says take 2 seconds longer so we have 48+2 =50 seconds to cover 1 km or 1000 m as we have formula like this: V*t = d so we have v*50= 1000 Or v= 1000/50 = 20 m/s

NOW convert to Km/h : 20 * 3600 (s) /1000 m = 72000/1000 = 72 km/h

Please confirm if this is true Thanks

I'm not Bunuel but I don't see anything wrong in the approach.

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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31 Oct 2015, 12:42

a tricky one, that requires to read attentively what is asking so the car travels 75km/hr. to cover 1 km, it would travel at 1/75 hours or 1/75 * 60 * 60 seconds. Simplify 75 and one of 60 by 15, and get 1/5 * 4 * 60 -> simplify again 5 and 60 and get 1*4*12 = 48 seconds. We know that the car makes 1 km in 48 seconds.

we know that the car spends 2 seconds more, and this is 48+2 seconds, or 50 seconds. We have the distance, and we have the time. We can solve for rate: R = Distance/Time = 1km/50sec, for the sake of getting rid of 50 sec in the denominator, divide 1km by 50 and get 0.02km/second now, 1 hour has 3600 seconds. Multiply 3,600 sec by 0.02km or 36*2 = 72km/h.

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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31 Oct 2015, 20:14

faster car travels 1 kilometer in 1/75 hour slower car travels 1 kilometer in 1/75+1/1800 hour 1/75+1/1800=25/1800=1/72 hour inverting time and rate, slower car travels at 72 kph

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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01 Nov 2015, 22:13

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Expert's post

RadhaKrishnan wrote:

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

If I had only 10 seconds to read the question and mark, I would mark it as (B) 72. The reason is the kind of numbers we are dealing with. 1 hour has 3600 seconds (which is a multiple of 2, 3 and 5, 18 etc). 75 km is again a multiple of 3 and 5. Getting 71 or 73 out of these numbers wouldn't really work out. It will be unusual to get decimals. So my best blind guess would be 72.

If I did have some time, I would use the unitary method on it:

Case 1: 75 kms in 3600 secs 1 km in 3600/75 = 48 secs

Case 2: In (48+2) = 50 secs, it covers 1 km In 3600 secs, it will cover (1/50) * 3600 = 72 km

Re: A car traveling at a certain constant speed takes 2 seconds [#permalink]

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02 Nov 2015, 01:13

RadhaKrishnan wrote:

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

Variables given :

d1= 1 km

d2=1 km s2=75

Variables expressed in terms of another:-

t1 = t2+ 2/3600

To find :

s1=?

We need to form two equations based on the above:

We know distance travelled is the same. So equate distances.

s1*t1=s2*t2 s1*(t2+2/3600)=75*t2 --- (1)

We also know the values of d2 and s2. So we can find t2. d2=s2*t2 i.e., 1=75*t2 => t2=1/75 hr

Substituting the value of t2 in (1), we get s1=72 km/hr _________________

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