Why would you use an LLM for this? A simple spreadsheet can do this sort of calculation easily and deterministically.
Also, the assumption of '3% interest' is wrong. There are records of stretches achieving 15% returns for several years and reaching 23% in 2007, for example.
https://www.bloomberg.com/news/articles/2005-01-11/harvard-l...
https://www.wsj.com/articles/SB118771455093604172
This was 2 minutes of old school search, no LLM needed.
Long term interest rates over hundreds of years are a lot closer to 3% than 15%. You can't extrapolate a few good years like that.
I don't think they intended to use AI, but tried to search instead and was presented with Googles AI summary / time wasted before the actual search results.