"How long do I need to keep money in an account for it to double if it grows at 8% a year" is an algebra problem. I suspect they're more common than you expect, you're just used to it.

That might have been a useful project back when we actually had interest rates :)

You still pay a premium because of default rates. The better your credit score the lower the predicted default rate and the lower the interest rate.

Low interest rates on loans are actually a pretty bad idea to stimulate an economy because the majority of people do not have access to them. If you gave people a $1000 coupon that lets them borrow $1000 at 0% interest regardless of the default rate you are going to see a bigger impact on inflation rates. The benefit is that people only take the loan if they actually spend it, therefore this is superior to helicopter money. If they default that's fine because then it turns into helicopter money which is still better than more loans for houses.

I don't know if the current algebra curriculum is teaching people how to solve real problems though.