The formula for the amount of a monthly payment A on principal amount P at interest rate i loaned for t years is given by
... A = P(i/12)/(1 -(1 +i/12)^(-12t))
Filling in the given values, this is
... A = $204000(.05/12)/(1 -(1 +.05/12)^(-12·20)) ≈ $1346.31