Given,
National Income $\left(\mathit{Y}\right)=1000$
Marginal propensity to save (MPS) $=0.20$
Autonomous consumption expenditure $=100$
MPC $\left(\mathit{c}\right)=1-$ MPS $=1-0.20=0.8$
As we know in equilibrium,
Since, $\mathrm{Y}=\mathrm{C}+\mathrm{I}$
We $\mathrm{C}=\mathrm{C}+\mathrm{cY}$
$\mathrm{Y}=\mathrm{C}+\mathrm{cY}+\mathrm{I}$
$1000=100+0.8\left(1000\right)+\mathrm{I}$
$1000=900+\mathrm{I}$
$\Rightarrow \mathrm{I}=100$
Therefore, investment expenditure is $\mathrm{₹}100$ .