On the earth, a stone is thrown from a height in a direction parallel to the earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?

For both the stones

Initial velocity, u = 0

Acceleration in downward direction = g

Now, h = {ut+{1\over 2}\ gt^2}

⟹    h = {0 +{1\over2}\ gt^2}

⟹   h = {{1\over2}\ gt^2}

⟹  t = \sqrt{2h \over g}

Both stones will take the same time to reach the ground because the two stones fall from the same height.