If someone deposited $8,000 into a savings account paying 5.25% ordinary interest, how long would it take for the investment to grow into $10,000? What is the equation for this... this is for a business math class.

What do they mean by "ordinary" interest? Does that mean non-compounding, like simple interest? I'm assuming simple interest in my answer. If this is not correct, please post back.

It's easiest to start by figuring out what interest is. If you put in $8000 and it grew to $10,000, how much is your interest earned?

Then I = Prt
where I = interest dollar amount
P = principal
r = rate
t = time

If you don't mind a little algebra, you can just solve for t.

Otherwise, we can twist this into:

t\,=\,\frac{I}{Pr}