take g=10m/s^2,it reaches the ground in the end

First you will need to find the velocity at which it reaches the ground, then you can find the average velocity.
How do you think you could tackle the problem?

Um... actually, the definition of average velocity is the total displacement (or distance) covered divided by the total time. So, here, you have the total distance, 125 m and you only need the total time. This can be obtained from the formula:

s = ut+ \frac12 at^2

s is the distance covered,
u the initial velocity,
t the time taken and
a the acceleration.

Once done, use the definition of average velocity:

Average\ velocity=\frac{Total\ Distance}{Total\ Time}

Unknown008, yes that is true but since it is uniform acceleration, the average velocity is also the average of the initial and final velocity, or half the final velocity if starting from rest.