(IS) Y = C(Y-T) + I(R - Pi) + G
<=> Y = O.9(Y - Y/9) + 1/(100(R-Pi)) + G
<=> Y = 9/10 * 8/9 Y + 1/(100R) +G
<=> Y = 0.8Y + 1/(100R) + G
<=> Y = 5G + 1/(20R)
<=> (Y - 5G)*R = 1/20
<=> R = 1/20 * 1/(Y-5G)


(LP) M/P = L(Y,R)
<=>  M/P = 0.8Y + 1/(100R)
<=> (M/P-0.8Y)*R = 1/100
<=> R =  1/(100 * M/P-80*Y)


(IS) = (LP)
<=> 20Y - 100G = 100 * M/P - 80 Y
<=> Y° = M/P + G