the main formula is
M= P (i(1+i)^n) / (1+i)^n) - 1
M: monthly payment
P: mortgage principale= $20,230
i=montly interest, 11.5%=11.5/100=0.115, i= 0.115/12=0.009
n=number of months=12x20(years)=240 months
M= $20,230(0.009(1.009)^240) / (1.009)^240-1=$20,230 x 0.0077 / 8.58-1
M=$20.57
