We assume that the bomb is small and falls under a parabola.

If the coordinates of the bomb are x(t) and y(t) then, measured from the ground directly below the bomber at the time of release then we have:

$x(t)=\mathrm{Vt}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}y(t)=-\frac{g}{2}{t}^2+H$
The bomb will have reached the dam at time D/V. At this point we require that the y coordinate of the bomb is between 0 and the height of the dam. This gives the inequality.

For the second part, the bomber can only just hit the target. As maximum speed and height and minimum distance the bomb will strike the dam 77.5 cm above the base.