19.56% is 19.56/12=1.63% per month=0.0163 as a fraction.
The growth after 3 months is 1.0163^3=1.0497 approx. Therefore the amount owing would rise to 1.0497*315.95=331.65.
If the monthly rate had been 1.5% the growth would have been 1.0457 and the amount owing 330.38.