The trajectory of projectile, projected from the ground is given by $$y=x-\frac{x^{2}}{20}$$. Where $$x$$ and $$y$$ are measured in meter. The maximum height attained by the projectile will be.

Two projectiles A and B are thrown with initial velocities of $$40 \mathrm{~m} / \mathrm{s}$$ and $$60 \mathrm{~m} / \mathrm{s}$$ at angles $$30^{\circ}$$ and $$60^{\circ}$$ with the horizontal respectively. The ratio of their ranges respectively is $$\left(g=10 \mathrm{~m} / \mathrm{s}^{2}\right)$$

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R

Assertion A : When a body is projected at an angle $$45^{\circ}$$, it's range is maximum.

Reason R : For maximum range, the value of $$\sin 2 \theta$$ should be equal to one.

In the light of the above statements, choose the correct answer from the options given below:

A child stands on the edge of the cliff $$10 \mathrm{~m}$$ above the ground and throws a stone horizontally with an initial speed of $$5 \mathrm{~ms}^{-1}$$. Neglecting the air resistance, the speed with which the stone hits the ground will be $$\mathrm{ms}^{-1}$$ (given, $$g=10 \mathrm{~ms}^{-2}$$ ).