The average speed of the bullet as it travels through board,
[tex]v=\frac{350 m/s+210 m/s}{2} =280 m/s[/tex]
As the speed of the bullet decreases linearly in the board, therefore the time of the bullet travels through the board is
[tex]t= \frac{distance}{v_{ave} }[/tex]
As the board is 4.0 cm ( 0.04 m) thick, therefore
[tex]t= \frac{0.04 m}{280 m/s} =1.43 \times 10^{-4} s[/tex]
Thus, the time taken by the bullet to pass the board is [tex]1.43 \times 10^{-4} s[/tex].
