Velocity problem? Please,show me the solution.

Question

A young man gave his best in tryouts day. It was on the 100m dash that he got the coach attention. Details of his run show that on the first 16m it took him 4s to achieve his fastest speed. He then maintained his fastest speed up to the finish line.

a) what is his speed on 3rd sec?

b) what is his average velocity from third to 7th second?

c)how long did it take him to finish the entire run?

Could you please show me how to solve it? These are the answers:a-6 mps b-10 mps c 10.5 s. Thank you.

Answer 1

a) what is his speed on 3rd sec?

0.5at^2 = 16

a=2m/s^2

speed= at

=2 x 3

=6m/s

b)

speed on 4s onwards = 2 x 4 = 8m/s

average velocity = (8 x 4 + 6)/ 5

=7.6m/s

c)s=ut

84=8t

t=10.5s