hire-purchase-calcs.js

var p = 25000, // principal amount
    i = 0.06, // annual interest rate over the life HP
    n = 60, // number of periods in life of HP
    y = 12, // number of payment periods per annum
    t = true, // if payment is to be made in ADVANCE
    r = 1 + ( i / y ), // interest rate per period = 1.005
    S_n = ( (r^n) - 1 ) ) / ( r - 1 ), // limiting sum of payments = 69.77
    repay; // repayments per period

if ( t ) S_n *= r; // = 70.12

repay = (p * ( r^n )) / S_n; // = 480.92