Use this when you need the exact interest rate that grows a principal to a target amount over a set number of years.

Use this when you need the exact interest rate that grows a principal to a target amount over a set number of years. type="compound" (default) uses the closed-form nth-root formula for the given compounding frequency; type="simple" uses linear growth. Requires target greater than principal and all values positive. Returns the annual rate as a percent plus the interest earned and the growth multiple; compoundingPerYear is null for simple interest. Deterministic: same input, same output. Example: principal=1000, target=2000, years=10, compoundingPerYear=12 -> ratePercent=6.9515, growthMultiple=2. Prefer this over trial-and-error.