Hi LUANDATO,

We're told that 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. We're asked at what speed, in km/hour, the car is traveling.

Your 'first step' - to convert km/hour into seconds/km is perfect. Since there are 3600 seconds per hour, it would take 3600/80 = 45 seconds to travel 1 km when traveling 80km/hour.

We're told that it takes 5 seconds LONGER to travel that same distance, so we now have to add those 5 seconds and 'work backwards' to find the speed...

(45+5) = 50 seconds to travel 1 km

Since there are 3600 seconds per hour, then we would end up traveling 3600/50 = 72 km in one hour. Thus, the speed is 72 km/hour.

Final Answer: B

GMAT assassins aren't born, they're made,
Rich

LUANDATO wrote: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/h. At what speed, in km/h, is the car traveling?

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

If a car is traveling at 80 km per hour, then the number of seconds (x) it takes to travel 1 km can be calculated using the following proportion (note that 1 hour = 3,600 seconds):

80/3600 = 1/x

80x = 3600

x = 45 sec

Thus, it takes 45 seconds to travel 1 km if the speed is 80 km/h. Since the car actually takes 5 more seconds (i.e., 50 seconds) to travel 1 km, its speed (r) can be calculated using the following proportion:

r/3600 = 1/50

50r = 3600

r = 72 km/h

Answer: B