(a) Distance covered in $5^{th}$ second
$S_{5^{th}} = u + \frac{a}{2}(2n - 1) = 0 + \frac{a}{2}(2 \times 5 - 1) = \frac{9a}{2}$
and distance covered in $5\ second$,
$S_{5} = ut + \frac{1}{2}at^{2} = 0 + \frac{1}{2} \times a \times 25 = \frac{25a}{2}$
$\therefore\frac{S_{5^{th}}}{S_{5}} = \frac{9}{25}$