if you earn 6 percent per year on your bank account, how long will it
take an account with $100 to double to $200?

Hi drew1952,

Assumed that the interest is payed once per year, it will take you twelve years:

100 x 1,06^12 = 201,2196471835550329409536

Kind regards,
paul_b_18-ga

This is a perfect example of the "Rule of 72".

If you divide the interest rate into 72, it tells you the number of
years until your money doubles.

72 / 6 = 12 years 
(as Paul calculated with a different method... but I can't pass up the
chance to explain the amazing Rule of 72)