What the risk-free rate should be ?

Teacher says "Assume the average cash rate will be the return on the risk free asset." Now I need to calculate portfolio of two risky assets & sharpe ratio Sharpe Ratio of a portfolio = Average Portfolio Return - Risk Free Rate Standard Deviation of the Portfolio Assume we have Asset A & B, they consist of a portfolio average portfolio return are based on Monthly average continuous return std.dev of portfolio are based on Monthly average continuous return I wonder what's the risk-free rate should be? we have Average Monthly cash rate Average Cash rate per annum Average Monthly average continuous return for cash rate Which one is true