You invest $6500 into a mutual fund that is expected to earn 6% per year. How long will it take the fund to be worth $13,000?

This is a compound interest problem so you cannot use the simple interest formula. Effectively the total amount is being compounded each year by 6%. There is a formula for compound interest but in this simple example you can work this out recursively.

year 1 = 6500

year2 = 6500*1.06 = 6890

year3 = 6890*1.06 = 7303

year4 = 7303*1.06 = 7741 ...

etc ...

year12 = 11604*1.06 = 12338

6% of 6,500 is 390. It will take over 16 years to reach 13,000. 390 * 16.795 = 6,500.05

So just under 17 years.