We have, $Y=C+I$ where, $I$ is investment expenditure which is given as 100. $Y$ is income which is given as 1200 and, $C$ $=\overline{C}+cY$ here, $\overline{C}$ is autonomous consumption expenditure is marginal propensity to consumer, $c=1$ - marginal propensity to consume (mps) so, $c=1-0.20=0.80$. Thus, putting the values in the equation, $Y=\overset{-}{C}+cY+I$ or, $1200=\overline{C}+$ $0.80 \times 1200+100$ or, $\overline{C}=1200-1060=140$. Thus, autonomous consumption expenditure is 140 .