I was taking a money management quiz from the MoneySmart government website. This was one of the questions: Question 4. If Laura deposited $20 a week in a high interest savings account with an interest rate of 6% for her newborn son, how much money would he have when he turns 21? I did a calculation on Excel assuming no tax and it gave the correct answer (about $43,000). However, in reality, wouldn't regular taxes severely reduce the final amount to well below $43,000?

The reality is, you can't extrapolate without including tax And inflation. If you saved, and once a year invested your saving for 21 years. The formula is: lump sum in todays buying power= X$yearly*(1+((interest rate-inflation)*tax))^(no of years) X$yearly=$20*52= $1040 Tax(assume a simple 15%)= 0.85 Interest rate= 0.06 Inflation( assume a flat rate 3.5%)= 0.035 No of years= 21 Lump sum= 1040*(1+((.06-.035)*.85))^21 = 1040*(1+(.025*.85))^21 = 1040*(1.02125)^21 = 1040*1.55515421 = $1617 in Year 1's buying power after 21 years. In other words Tax and Inflation count! They must be included in your calculations. Your buying power isn't even doubled in 21 years. Clearly not a good outcome. Tax and inflation are killers. Johny.

Thanks! Yeah, that's what I thought after doing the question. Although I guess that makes the question too complicated for a simple quiz. Nevertheless, they probably should have given an asterisk note about tax and inflation.