What is the formula to calculate interest on accounts receivable at 1.5% monthly the beginning balance is 20191.64

Sounds like a homework question...

% X balance.

Interest is equal to Principle times Rate times Time. You have your principle and rate, which is the monthly rate, so your time will be 1/12 for one month out of 12 months in a year. For more than one month just take your monthly interest times the number of months, assuming your principle stays the same.