Calculate investment expenditure

Calculate investment expenditure from the following data about an economy which is in equilibrium:
National Income =1000
Marginal Propensity to Save = 0.20
Autonomous consumption expenditure =100

Given, Y = 1000,
MPS = 0.20, $\overline{C}$ =100, I= ?
b or MPC = 1 - MPS = 1-0.20 =0.80
Y=C+I
or Y =$\overline{C}$ +bY + I
1000 =100+050x1000 + I
I = 1000-900,
I = 100