It is Charcoal which is black when we buy it or when not being used. It gets red in appearance when put in use means to say when burning. And eventually when it's been consumed by fire or after getting burned out or used up, changes into ashes which look grey in color.
A man reaches his office 20 min late, if he walks from his home at 3 km per hour and reaches 30 min early if he walks 4 km per hour. How far is his office from his house ?
Let distance = x km. Time taken at 3 kmph : dist/speed = x/3 = 20 min late. time taken at 4 kmph : x/4 = 30 min earlier difference between time taken : 30-(-20) = 50 mins = 50/60 hours. x/3- x/4 = 50/60 x/12 = 5/6 x = 10 km.
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