Rough notes that may precede or wrap a C prog + web page later.
At present not linked to rest of my pages.
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Those interest rates will presumably be compound not flat.  That
means one charges the 365 th root of the yearly rate as the daily
rate.  One can do that using log tables or a computer.  There may
even be web sites that offer such calculators.

Very few people really understand compound rates: Every employee
in City of London Moorgate branch of Leeds Permanent Building Society
did not either, till I got the manager (or maybe deputy manager),
who told staff I was right, not off balance, & confirmed something
they didn't know & virtually no British public knew: That the montly
interest repayments on mortgages were calculated only yearly, thus
all building society loan rates were misleading lower than true
rates.

That was back around the early 1980s. Things might have changed.

(I had used some algebra to check my mortgage repayments & had found
I was paying more than I thought I should have.  Maybe they've
changed it since (& maybe not), but it wasn't a fair RPI/APR wjatever
they call it, comparable rate).

True rates are eg what credit card companies charge on loans: daily
interest on the capital & interest of previous day.