A projectile is thrown with a speed u at angle ‘θ’  above an inclined plane of inclination 'β'. The angle ‘θ’ at which the projectile is thrown such that it strikes the inclined plane normally is

Time of flight=T=2usinθgcosβ                  _ _ _ _(1) ‘If it strikes the inclined plane normally  then component of velocity along the plane =vx= 0 we know,     vx=ux+at  subtitute vx=ucosθ−(gsinβ)t t=ucosθgsinβ                                            −−−−−− 2  Equating (1) & (2)  2usinθgcosβ=ucosθgsinβ 2tanβ = cotθ θ= cot−1(2tanβ)