We assume that we have two and only two contries: Albania and Bulgaria. There is complete free trade between the two countries.
The aggregate investments in Albania are given by: $$I^A=A-ar$$ where $A, a >0$ and r real interest rates.
The aggregate investments in Bulgaria are given by: $$I^B=B-br$$ where $B, b >0$ and r real interest rates.
They have total savings $S^A$ and $S^B$ who are independent of the interest rate.
Now I have to derive the condition that Albania is a net exporter.
My though so far is that I have to use that BNP(Y): $$Y=C+I+G+NX$$ and $$S_A=publicsavings+privatesavnings=(T-G)+(Y-T-C)=Y-C-G$$ and that $NX>0$. But when I use these can only find an expression of the form $S_A>f(A,a,r)$. But I think I have to find an expression of the form $S_A>f(A,a,B,b,r)$. Can someone help me?