Country inside PPC can:

The quantity theory of money relates the money supply to price levels in an economy, focusing on the long-term relationship between inflation and the quantity of money in circulation. It does not directly address the relationship between nominal rates, expected inflation, or real interest rates, thus making it irrelevant to the question asked.

The money multiplier refers to the process by which an increase in the monetary base leads to a larger increase in the money supply through banking activities. While it is related to monetary policy, it does not involve the calculation of real interest rates or the impact of inflation on nominal rates, which is the focus of the Fisher equation.

The Keynesian cross is a model that illustrates the relationship between aggregate demand and aggregate supply in an economy, particularly in the short run. It does not concern itself with the calculation of real interest rates or inflation, making it unrelated to the concept described in the question.