numericals

when cost function and investment function is given how to find equilibrium in general?
and in question below Consumption and investment function in a closed economy without government are given
respectively by C = 100 + .7Y and I = 200 + .1Y .
how to find equilibrium?
such kind of sums are there in material/book ???

 

To find the equilibrium level of national income, you need to set aggregate demand (C+I+G+X-M) equal to aggregate supply (Y). So if C=10+0.8Y, I=20,G=20, X=10 and M=0.1Y, then aggregate demand (or planned demand, E)= 60+0.7Y. So, setting this equal to Y, 60+0.7Y=Y, implies that Y must be 200.