Projectile on an inclined plane

Let's consider that the particle did not hit the inclined plane normally.So it must have some horizontal and vertical velocity.Since the collision is elastic so after striking the plane it would have the same horizontal and vertical velocity,i.e. it would have some velocity up the incline.In that case the particle would have never retraced back it's path.It would have bounced upwards.So the only possibility is to hit the plane normally.
Then ,u*cosθ=(gsin45)*t............(1)[no horizontal velocity while striking].
u*sinθ=(gcos45)*(t/2)...............(2)[at the uppermost point of it's flight vertical velocity is 0].
Now,dividing we get cotθ=2.
NOTE:Here horizontal and vertical velocities are with respect to the inclined plane.