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Speed or magnitude of velocity at projection is u. Its components are Vx and Vy horizontally and vertically.    Angle of projection with horizon is Ф.

Vx is constant = u cos Ф
Vy is highest at the time of projection and is zero at the top of parabolic path and again maximum when it reaches the ground.
Velocity V = √(Vx² + Vy²)    as Vx is constant , V is maximum when Vy is maximum at the point of projection and V is minimum when Vy is zero
Vmax = V itself                Vmin = Vx = V cos Ф
Vmax/Vmin = 2 =>  Cos Ф = 1/2   =>  Ф = 60 degrees

If the maximum projectile is doubled the minimum velocity, the angle of projectile is 60 degree