$S_{2.5}-S_2=?$ (distance travelled in $5^{\text {th }}$ half second)
$S_{2.5}=u t+\frac{1}{2} a t^2$ $a=2 \,ms ^{-2}$
$\Rightarrow S_{2.5}=\frac{1}{2} \times 2 \times(2.5)^2=6.25 \,m \quad(\because u=0)$
$S_2=\frac{1}{2} \times 2 \times 4=4 \,m$
So, $S_{2.5}-S_2=2.25 \,m$