Shortcut : $S=\frac{1}{2} \frac{\alpha \beta}{\alpha+\beta} T^2$

$\alpha \rightarrow$ Acceleration

$\beta \rightarrow$ Deceleration (magnitude only)

$T \rightarrow$ Time of journey

$S \rightarrow$ Distance travelled

Given, $\alpha=0.2 \,ms ^{-2}$

$\beta=0.4 \,ms ^{-2}$

$T=$ half an hour $=30 \times 60 \,s =1800 \,s$

$S=\frac{1}{2} \times\left(\frac{0.2 \times 0.4}{0.2+0.4}\right) \times(1800)^2$

$\Rightarrow S=216000 \,m$

$\Rightarrow S=216 \,km$