Question

If one airplane can travel at a maximum speed of 600miles per hour and another airplane at max speed of 720 miles per hour, then how many more miles can the faster airplane travel in 12 seconds than the slower airplane?A. $\dfrac{1}{30}$ B. $\dfrac{2}{5}$ C. 2D. 30

Hint: We have two planes at speed of 600miles per hour and at 720 miles per hour, Look carefully speed is given in hours and distance asked in seconds so we will first convert speed in seconds and then we just have to find the difference of distance travelled by both plane in 12 seconds

Given two planes at speed of 600miles per hour and at 720 miles per hour
And we are asked to find extra distance in miles by the faster airplane in 12 seconds than the slower airplane
So first we will convert speed in terms of seconds by using formula
$1hour=60\times 60\sec onds$
$600\dfrac{miles}{hour}=600\dfrac{miles}{60\times 60\sec ond}=0.167\dfrac{miles}{\sec ond}$
Similarly $720\dfrac{miles}{hour}=720\dfrac{miles}{60\times 60\sec ond}=0.2\dfrac{miles}{\sec ond}$
Now the distance travelled by both planes in 12 seconds is
$0.167\dfrac{miles}{\sec ond}\times 12\sec onds=2.004miles$
$0.2\dfrac{miles}{\sec ond}\times 12\sec ond=2.4miles$
To find how many more miles can the faster airplane travel in 12 seconds than the slower airplane, this line basically means that we have to find extra distance travelled by faster plane than slower one
It means we just have to subtract both the distances
Distance by faster plane – Distance by the slower plane
$2.4miles-2.004miles\simeq 0.4miles$
$0.4miles=\dfrac{2}{5}miles$

So, the correct answer is “Option B”.

Note: We can also solve by considering relative speeds, it means we will do it by subtracting both speeds at initial and then multiply it with the time given and then at last we will put hour in seconds, for example
Relative speed $v=720-600=120\dfrac{miles}{hour}$
Now multiple v with 12 seconds $s=12\times 120=1440\dfrac{miles}{hour}\sec onds$
$s=1440\dfrac{miles}{3600}=0.4=\dfrac{2}{5}$