Time of flight T=u×sin30/g

T=10s

10=u×sin30/g

u=10 ×g÷1/2

u=20×g=196m/s

The angle made by velocity vector with the horizontal  after 1.5 s is zero

Explanation:

Initial velocity of the body u = 30 m/s

Angle of projection = 30°

Horizontal component of the velocity = ucos30° = 30 × √3/2 = 15√3 m/s

Vertical component of the velocity = usin30° = 30 × 1/2 = 15 m/s

The acceleration due to gravity is acting only in the vertical direction

Therefore, only the vertical component of the velocity will change

Using the first equation of motion

If the vertical component of the velocity after 1.5 seconds is v then

v = u - gt

v = 15 - 10 × 1.5

= 15 - 15

= 0

Therefore, after 1.5 seconds, the body will have only horizontal velocity

Hence the Angle made by velocity vector with the horizontal = 0

Hope the answer is helpful.