We know that in equilibrium,
$\mathrm{Y}=\mathrm{C}+\mathrm{I}$
where,
I is investment expenditure, given as 300
$\mathrm{Y}$ is national income, given as 1500 and, $\mathrm{C}=\overline{\mathrm{C}}+\mathrm{cY}$.
here, $\overline{\mathit{C}}$ is autonomous consumption expenditure, given as $300,\mathit{c}$ is marginal propensity to consume.
Thus, putting the values in the equation, we have $\mathit{Y}=\overline{\mathit{C}}+\mathit{c}\mathit{Y}+\mathrm{I}$.
or, $1500=300+\mathit{c}\times 1500+300$
or,$\mathit{c}=\frac{1500-600}{1500}=0.6.$
Thus, marginal propensity to consume is 0.6 .