Formula

Fixed Variables:
INT = 20% or 1.20
Term = 3 months or 6 payouts (15th/30th)

Total Loan = Principal x Int Rate 
Php 15,000.00 x 1.20 = Php 18,000

Loan Repayment Computation:
Total Loan / term
Php 18,000 / 6 = Php 3,000.00