Second, specifically:

That is: Y=C+I, Y= 100+0.75Y+20-2r, Y = 480-8r；;

LM:L=M，0.2Y-0.5r=50，Y=250+2.5r

Three. IS-LM model:

1, IS-LM model is an important tool for macroeconomic analysis and a theoretical structure to describe the relationship between product market and money market. In the product market, national income depends on the total expenditure or total demand level of consumption C, investment I, government expenditure G and net export X-M, while total demand, especially investment demand, is affected by interest rate R, which is determined by the supply and demand of money market, that is, money market will affect product market;

2. The national income determined by the product market will affect the money demand, thus affecting the interest rate, which is also the influence of the product market on the money market. It can be seen that the product market and the money market are interrelated and interactive, and the income and interest rate can only be determined in this connection and interaction.

3. Describe and analyze the theoretical structure of the two markets, which is called IS-LM.

4. The model needs the following two conditions:

1)I(r)=S(Y) Yes, the investment is economical.

2) M/P=L 1(Y)+L2(r) is LM, and liquidity preference-money supply.

Where I is investment, S is savings, M is nominal money, P is price level, M/P is real money, Y is total output, and R is interest rate.

5. The intersection of the two curves indicates that the product market and the money market reach equilibrium at the same time.

6.IS-LM model is an important tool for macroeconomic analysis and a theoretical structure to describe the relationship between product market and money market.