A car traveling at a certain constant speed takes 5 seconds longer to

Question

A car traveling at a certain constant speed takes 5 seconds longer to travel 1 km than it would take to travel 1 km at 80 km/hour. At what speed, in km/hr, is the car traveling?

A. 70
B. 72
C. 74
D. 75
E. 78

Bunuel · Accepted Answer

Time to cover 1 kilometer at 80 kilometers per hour is 1/80 hours = 3,600/80 seconds = 45 seconds;

Time to cover 1 kilometer at regular speed is 45 + 5 = 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.

Thoughtosphere · Answer

Hi Please let me know, whether my approach is correct and whether we can have a faster method to get the right answer

It takes the car 5 sec. longer to travel 1 KM than it would take, had it traveled at 80Kp/Hr
This would mean that it would take the car 200 sec extra had it traveled 40KM
that is equivalent to 10/3 minutes extra to 30 min. it would have taken at 80Km/Hr

Thus it would take 30 + 10/3 minutes to cover 40 Km or 100/3 min = 100/(3*60) hrs

Thus the current speed would be 40*3*60 / 100 = 72 Km/Hr

choice B

Bunuel · Answer

This is a copy of GMAT Prep question discussed here: a-car-traveling-at-a-certain-constant-speed-takes-2-seconds-126658.html

manpreetsingh86 · Answer

time taken to travel 1km at 80km/h = (1/80)*3600 =45 seconds

therefore, first car takes 50 sec to travel 1 km. therefore its speed = (1/50)*3600 =72km/h

adiagr · Answer

Car travelling at 80kmph covers 1 km in (1/80) hrs =  \frac{60*60}{80}  = 45 sec.

The other car takes 5 sec more. So the time taken by that car is 50 sec. Distance is 1 Km. So its speed is (1 Km / 50 sec) or

\frac{60*60}{50}  = 72 kmph

B is the answer.

gracie · Answer

1k/80kh=1/80h
5 seconds=1/720h
1/80h+1/720h=1/72h
1k/(1/72h)=72kh

energetics · Answer

(80km / 1hr) * (1hr / 60^2 sec) = 1 km / 45 sec
This is the faster rate, t-5 = 45 sec, so time to travel 1 km at the slower rate is t=50 sec
(Total time of 1 h to travel) 60^2 sec / (time to travel 1 km) 50 sec = 72 km rate of travel in 1h.

Another way, ratio of time taken to travel 1 km is 45/50 = 9/10 
9/10 * original rate of 80 km in 1 hr = rate of 72 km / 1 hr

**A good way for me to think about it is that for each 9 iterations of the first rate (at 1 km/45 sec), we need 10 of the 2nd, slower rate (1 km/50 sec) to reach the same distance (since 2nd rate has to catch up by 5 sec every time, after 9 it's a whole extra unit of work).

bumpbot · Answer

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