The formula for monthly repayment, M, that
you may not use for the HSC is:
where P = $200000,
n = 20 x 12 = 240 compounding periods and
r = .06/12 = 0.005 = interest rate per compounding period
so that
i.e. $1432.86 per month.
Over the life of the loan, Pedro would have paid: 240 x $1432.86 = $343,886.40
Therefore total interest paid = $343,886.40 - $200,000 = $143,886.40.
In the HSC, you are required to derive the solution, which takes too long to type out.