Math problem

I know, I know. I am not the mathematician in my household.

However, the business badge requires that I calculate how much money I'd need to invest, at 5% per year, to earn back my weekly allowance. Now, here's the situation: since I am a self-sufficient adult, I don't get an allowance (sadly). And even though you are nice people on the internet, I would prefer not to be 100% open about my paycheck and savings habits. So you'll have to bear with me for a moment.

Instead, I'm going to tell you how much I've spent this week on non-household expenses (since we will assume our 1911 Boy Scout was probably not running exciting errands like purchasing dishwashing soap or canned olives), and we'll assume that that's my "allowance." (I feel really uncomfortable with the allowance term, maybe moreso than I should, because talking about it makes me feel like my money is not my own. But I digress.)

This week:
$3.75 for hot chocolate with Wendy last week
$15 for Greek take-out (spanakopita, mostly) Friday night
$6 for a glass of wine on Saturday night (happy birthday, EFS!) (I know a scout is temperate. But a scout is also under 21.)
$3.50 for a coffee and chocolate croissant yesterday
For a grand total of $28.25, all of it spent on food. Go team.

Okay, if there are 52 weeks in a year, this means I'd need $1,469. Okay. First, allow me to catch my breath because that seems like an alarming amount of money. And I haven't even factored in the times I do things like, you know, buy a book. Or a postage stamp.

(Please note, by the way, that I am doing this calculation without benefit of the internet, just what I remember from high school math. Please note, also, that I am not asking John to check my work, because that seems like it would be cheating. If you find a mistake, though, feel free to make me feel like an idiot.)

I'll use the formula interest = principal * rate * time, with principal and interest switched around to give me the formula 1/P = (RT)/I. For the sake of simplicity, I'll take the inverse of each (is this the correct term? I have no idea!), giving me the final formula P = I/RT

I'm looking for the principal, so I can say P = $1469 (the amount of interest I'll need!)/(0.05). Notice that, since I'm only doing this over a single year, T = 1, so it drops out. I love that. (When I learned this in math class when I was a kid, it was like a magic trick.)

Simplify the whole shebang to come to the ultimate conclusion that, in order to earn out my allowance in interest, I'll need to invest $29,380 at 5% per year. If you can tell me where to put my money at 5%, I'll be pretty satisfied.

Yay!