## Calculating Interest: How My Late Grandmother Got My Gears Turning

1/19/2013

Grandma made PLENTY of popcorn when we visited.
Consider this example, that might very well appear in a math textbook:

How long would you leave a \$50 investment in a bank, with 2% interest, compounded annually, to yield a balance of \$500?

Applicable, yes, but how enticing is it, really?

My grandmother passed away several years ago, and I miss her plenty. I'm certain my daughters would have enjoyed meeting her and hearing some of her stories, too. Or the way she could teach a new card game almost every time I went to visit her.

One of my favorite memories of visiting Grandma's house was how she would make enough popcorn to fill half a paper grocery bag, then keep it warm in her stove. And she was always generous in using salt on her popcorn, which I consider an extra perk.

Grandma
What really got me thinking, though, was when my parents told me they had found some savings bonds at Grandma's house and bank that were in my name, dating back to just after my birth. Grandma never brought it up to me, nor did she bring it up to my parents. This generous act on her part gives rise to the name of my blog: Good for Nothing (see blog post HERE about the relevance of the name). Grandma made a grand gesture with no expectation of recognition or grand reward in return. She truly did something good, for nothing in return.

The math teacher in me can't leave this alone though. Knowing the end balance of these savings bonds' values, I'm curious about what the interest rate was, but realize that Grandma kept adding to the collection of bonds, which would make this more of a recursive function.

Nevertheless, I greatly appreciate her generosity, even long after she left this world. It definitely makes for a great Act 1 in the 3-Act lesson format, and I'd love to figure out how to get through Act 2 to find the result in Act 3, but am uncertain as to where to go next.

This might be a better example to leave to rest though. Take a moment with your Grandma next time you are with her. She just might bless you with this sort of curiosity someday like mine did.

--Keltner--

## Scott Keltner

Math teacher and loving it. Tech-savvy because it's necessary. Husband and dad (only male in the house besides the dog).
Kansas, USA