Question 1

What is compound interest and how does it generate exponential growth?

Accepted Answer

Compound interest is interest calculated on the initial principal plus all previously accumulated interest from past periods. Over long time horizons, this reinvestment creates an exponential curve where interest earnings eventually surpass your cumulative cash contributions.

Question 2

How does compounding frequency (Daily vs Monthly vs Annual) impact APY?

Accepted Answer

More frequent compounding reinvests earned interest earlier, generating higher Effective Annual Rates (APY). For example, $10,000 at 8% nominal APR yields $800 annually with annual compounding, but $832.78 with daily compounding due to intra-year reinvestment.

Question 3

How does inflation erode future investment purchasing power?

Accepted Answer

Inflation reduces the real purchasing power of future cash. While your nominal portfolio balance may grow to $500,000 in 20 years, a 2.5% annual inflation rate reduces the real purchasing power to approximately $305,000 in today’s constant dollars.

Question 4

What is the Rule of 72 and how is it used for mental estimation?

Accepted Answer

The Rule of 72 provides a quick mental shortcut to estimate how many years it takes for an investment to double: divide 72 by your expected annual interest rate (e.g. at 8% annual return, an investment doubles approximately every 72 / 8 = 9 years).

Question 5

What is the mathematical difference between beginning vs end-of-period deposits?

Accepted Answer

Deposits made at the beginning of each period (annuity due) earn an entire additional period of compound growth compared to deposits made at the end of the period (ordinary annuity), resulting in noticeably higher lifetime yields over decades.

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