The answer to the question is  

CALCULATION:

As per the question, the body starts from rest.

Hence, initial velocity of the body u = 0.

The acceleration of the body is given as a .

The time taken by the body is given as t.

Let the distance covered by the body in time t is S.

From equation of kinematics, we know that-

                               ⇒

                                ⇒

The average velocity of a body is defined as the ratio of total distance travelled to total time taken.

Mathematically  

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