How do I use a Do Loop on an adjusted rate loan?

[matoma]

$85,000 federal student loan at annual interest rate of 7% and a payment of $500 every month. When the principal hits $50,000, the annual interest rate drops to 5%. When the principal gets down to $30,000, the annual interest rate drops to 3%. using a Do Loop how would I calculate how many years it would take to pay off the loan?

[Astounding]

Variations on this question have been asked quite a few times.

If you want help with this, you probably have to do a little more of the work.  For example, write a program that calculates the balance remaining after 1 month.  That way, it becomes apparent that you are not just looking for somebody to do your homework for you.

[mkeintz]

Here's the logic wrapped in a do loop:

data payments;
  principal=85000;
  do month=1 by 1 until (principal=0);
    *depending upon principal set yearly rate;
    *calculate one-month interest using principal and rate;
    *establish payment size (either $500 or current principal + interest, whichever is smaller);
    *using old principal, payment size, and calculated interest, calculate new principal
    *output a record, if wanted;
  end;
  * After the loop, from the value of MONTH, generate the number of years (it's a long time)
run;