(b) Time taken by the bomb to reach the ground is given by
$h_{OA} = \frac{1}{2}{gt}_{OB}^{2}$
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$we\ have\ \ \ \ \ t_{OB} = \sqrt{\frac{{2h}_{OA}}{g}}$
$given,\ \ \ h_{OA} = 80m,\ \ g = {10\ ms}^{- 2}$
$\therefore\ \ \ \ t_{OB} = \sqrt{\frac{2 \times 80}{10}\ 4s}$
$Horizontal\ velocity\ of\ bomb\ $
$v = 150\ {ms}^{- 1}$
Horizontal distance covered by the bomb
$AB = {vt}_{OB}$
$= 150 \times 4$
$= 600\ m$
Hence, the bomb should be dropped 600 m before the target.