The equivalent nominal monthly interest rate can be calculated by dividing the annual interest rate by the number of months in a year (12). The formula to convert an annual effective interest rate into a monthly nominal rate is as follows:

Nominal monthly interest rate = (1 + Nominal Annual Rate)^(1/12) - 1

Given that the effective annual interest rate is 4.2%, we first need to convert it to a nominal annual rate because the effective annual rate is already given. The conversion from effective to nominal annual rate is done by adding the expected inflation rate to the effective annual rate.

Nominal Annual Rate = Effective Annual Rate + Expected Inflation Rate

However, since no expected inflation rate is provided, we'll assume the nominal and effective annual rates are the same for this calculation.

Nominal Annual Rate = 4.2%

Now, to find the equivalent monthly nominal rate:

Nominal Monthly Rate = (1 + Nominal Annual Rate)^(1/12) - 1 Nominal Monthly Rate = (1 + 0.042)^(1/12) - 1

This will give us the monthly nominal interest rate.