There's no mention of whether simple or compound interest applies, so I'll start with simple interest=PTR/100=20000*20*6.5/100=26000. So the amount is 20000+26000=46000. 20 years = 240 months so the monthly payment is 46000/240=$191.60 per month.
Next, assume 6.5% compounded annually. The total rate applicable after 20 years is 1.065^20=3.5236, so the amount is $70,472.90, making $293.64 per month.
Next, assume 6.5% compounded monthly, that is 6.5/12=0.54167% per month. After 240 months this amounts to 1.0054167^240=3.65645, making $73,128.93 over 20 years, or $304.70 a month.