Question

When calculating simple interest, what must you do if you want to invest for months or weeks instead of years?
a.
Simple interest only pays yearly, so you should find another investment.
b.
The simple interest calculation does not use those units, so you should just calculate as normal.
c.
The units have changed, so the interest formula switches to I·t=P·r.
d.
Interest rates are annual, so you must convert the units into years.

Answer 1

d.

Interest rates are quoted per year, and the simple interest formula is I = P·r·t with t in years. So convert months or weeks to years (months/12, weeks/52) or adjust r accordingly (e.g., for 9 months use r·9/12).