How long will it take $1,000 to double when it is invested at (a)15%, (b) 13% or (c) 12%?

Let's see, first at 15%, that would be 72 divided by 15 =s 14.8 years. second at 13%, that would be 72 divided by 13 =s 5.53 yrs, and finally at 12%, that would be 72 divided by 12 =s 6 yrs. I used the rule of 72s.