March 15 .Feder sole merchanides for CHS400 in its retail outlet.

March 31. The customer used his Federer credit card. Ferderer added 1.5% monthly interest to customer's credit card balance.

Prepare journal entries for the transaction above.

I know the answer c is debit Account receivable 3, and interest revenue is 3.

And the way to do that is 400X0.15X15/30

But I don't get why it is 15/30. Does it relate to the number of dates? Then if it was 3/1~3/20, does it have to be 20/30??

Please explain the process, not only the answers.