You can put this solution on YOUR website!
Trick question? You don't earn interest when you borrow. You PAY interest. So... never?? :)
Now suppose you invested $600 at 10%, how long would it take you to earn $30?
Start with - how much do you make per year?
So you make $60/year. If you make $60/year, how long will it take to make $30?
So it take half a year. How many months is that?