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Explanation:

a )

According to graph the object moves with constant velocity of 10 m /s during first 8 s and then its velocity changes to - 5 m /s from 8 th second upto 12 th second .

Initial displacement = 8 m

displacement during 8 s = velocity x time

= 8 x 10 = 80 m

total displacement = 88 m

b )

displacement during period from 8 th to 12 th s

= - 5 x 4 = - 20 m

total displacement after 12 s

= 88 - 20 = 68 m

c ) average speed = total distance covered during 12 s / total time

= (80 + 20) / 12

= 8.33 m /s

average velocity = total displacement during 12 s  / total time

= 80 - 20 / 12  

= 60 / 12

= 5 m /s .

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