This is the boss. No calculator. Every rate on this page is 6%, every number is chosen to fit in your head, and every answer feeds the next: the flat benchmark, then the compounding account, then the gap between them, then the savings plan, then the doubling check. Write each line down — one early slip rolls all the way to the verdict.

Three nuggets you will need. First, 6% compounded semi-annually is 3% twice: multiply by 1.03, then by 1.03 again, and $1.03^2 = 1.0609$ — so a year of it pays 6.09%, not 6%. Second, two end-of-year deposits at 6% grow by a factor of exactly $2.06$, because the first deposit earns one year and the second earns nothing. Third, $72 / 6 = 12$: at this rate, money doubles in twelve years.

The household is asking a real question, and the last line is where you answer it. 5★ under par here is the chapter crown. Clock in.