An object is thrown vertically upward with a speed of 30 m/s. The velocity of the object half-a-second before it reaches the maximum height is

Velocity half second before maximum height = Velocity half second after maximum height (Return journey)

For downward journey : 

Initial velocity  u = 0 m/s   

We have  g = 9.8 m/s²

Time  t = 1/2​ s

Thus velocity after half second  v = u + gt 

v = 0 + 9.8 × 1/2​ = 4.9m/s