Since it states that 6% is compounded monthly, it can easily be converted to compounded annually:
[1 + 6/[12. 100]]^12 =1.005^12 =1.0616778 -1 x 100 =6.16778%- This is called the EFFECTIVE annual rate. 6% compounded monthly is called the "NOMINAL" annual rate. You took 6% as the "effective" annual rate instead of "nominal" annual rate, and found its "nominal" monthly rate by: 1.06^(1/12) =0.486755.