A person crosses a 600m long street in 5 minutes, what is his speed in km per hour?
A. 3.6
B. 7.2
C. 8.4
D. 10

Hint: We will convert distance to km using the concept that 1km = 1000m and then convert minutes into hours using the concept that 1 hour = 60 minutes. In the final step we will find the speed of the man using the formula, $\text{Speed=}\dfrac{\text{Distance}}{\text{Time}}$.

Complete step by step answer:
It is given in the question that a man crosses 600m long street in 5 minutes. We have to find the speed of the man in km per hour. To find the speed of the man in km per hour, we have to convert the given distance to km and also convert the given time to hour.
First, let us see the conversion of the given distance into km. We know that 1km = 1000m.
So, to convert 600m to km, we can write, 600m = x km.
$x=\dfrac{600}{1000}km$
x = 0.6km
Now, let us convert the given time into hours. We know that 1 hour = 60 minutes.
So, to convert 5 minutes to hours, we can write, 5 minutes = y hours.
$y=\dfrac{5}{60}hrs$
y = 0.083 hrs
Now we know that speed can be found out using the formula, $\text{Speed=}\dfrac{\text{Distance}}{\text{Time}}$. We have found out that distance = 0.6km and time = 0.083 hrs. So, on substituting these values in the formula of speed, we get, $Speed=\dfrac{0.6}{0.083}$.
$=\dfrac{6\times 1000}{83\times 10}$
$=\dfrac{6000}{830}$
$=\dfrac{600}{83}$
= 7.2 km/h
Therefore, we get the speed of the man crossing a 600m long street in 5 minutes as 7.2km/h.
Hence, option (B) is the correct answer.

Note: Students must make sure that the conversions are done properly as the final answer may change if the values are not converted properly. It must be noted that 1km = 1000m, so to convert m to km, we have to divide by 1000 and to convert km to m we have to multiply by 1000. And we know that 1 hour = 60 minutes, so to convert minutes to hours, we have to divide by 60 and to convert hours to minutes we have to multiply by 60.