Goldenbutterfly used a rough rule, and gave a precise answer, a more accurate but less precise answer (if you want to know the difference between accuracy and precision in math, please ask me) would be "11".
Let me show you how to work it out without using the rule.
This is the formula you need to solve to get the correct answer, you start with 1 dollar, the interest over n years is 1.065^n (100% + 6.5%, n times), and you want to end up with 2 dollars. All you need to do now is solve for n.
Get rid of the 1, it's useless:
take logarithm of both sides:
Use log rule to bring power out of the logarithm:
Rearrange logarithms:
Putting this into a calculatr, we get n = 11.008 years, which is quite a way off golden butterfly's answer, and is much more accurate. Goldenbutterfly, it's important to consider how accurate your answer is, and to put the right number of decimal places for the answer! '11' would have been much more accurate.
Now I have shown you how to work out the first one, see if you can work out the rest, they all work from the same principle, with a few simple complications thrown in.