Don't worry, your math teacher is absolutely right about this.
The way it's set up is that the 24 hours option always provides the double percentage of the 8 hours option. Why you ask?
You can see it is like this (with level 6 of Banking as an example):
- With the 8 hours option you do nothing for 8 hours and get 2% interest on your gold.
- With the 24 hours option you do nothing for 24 hours and get 4% interest on your gold.
Apparently the bank doesn't want to reward you for doing nothing for a longer period. It's either this or they would just like to see their customers more often, because it is still possible to put the gold back every 8 hours.
Here you can make 8h deposit 3 times per day and get 6% interest, but when you make one 24h deposit you get only 4%. I don't want to hurt your feelings, but the way it works in this game is very stupid. I can't find another word for this twisted logic
In the real life the bank acts just the opposite. Long time deposit gets much more interest.