We know that speed = dist/time 
or in this case,  200/24 m/s

In order to convert m/s in to km/hr, we multiply 3600/1000
= 200/24 x 3600/1000
= 200/24 x 18/5
= 30 km/hr

Hope that helps !!
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Sir how to come 3600\1000
In order to convert seconds into hours, we multiply 60 x 60 and to convert meters to kilometers, we multiply 1000. So that comes to 3600/1000 or 18/5.
And please dont call me sir, I am way younger than u :)
To find the speed km/h, we should convert the meter into km and sec in hour. And we should use the speed formula that is distance travelled/time and you get the answer as 0.5