Average speed =total distance travelled
                            total time taken
To specify the speed of an object we require only its magnitude.The speed of an object need not be constant.In most cases it will be non uniform.Therefore we describe the rate of motion of such objects in terms of their average speed.
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Average speed= distance/ time = 600/30=20 m/s
if you travel at 300 metres for 30 seconds then the actual speed is 10 m/s.
but the average speed is  m/s. 
the difference is 10 m/s.
so its actual speed differs from its average speed
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