calculate APR using cumulative interest?

[Guest]

How would I calculate the effective APR on a loan who's APR and payment size
changes? I know the cumulative interest and the original balance... Is
there a formula that I could use?

 

[Fred Smith]

Cumulative interest is insufficient to calculate APR, because you don't know
when the interest is charged.

If you want to calculate the rate on a loan with variable payments, you need the
XIRR function. Build a table of cash flows and the dates they occur, and XIRR
will calculate the APR for you.