Let the velocity of the stone when it is at height \( h \) above \( A \) be \( v \).
At height \( h \) below \( A \), the velocity becomes \( 2v \).
Using the equation of motion: \( v^2 = u^2 + 2gh \), where \( u = 0 \) at the maximum height, we can use the principle of conservation of energy. We calculate the height using the relationship between the velocities at different heights, which gives the greatest height as \( \frac{6h}{5} \).