Half-life problem...?

There is a simple way to go about these problems, but I wouldn't recommend using it all the time as it is kind of a shortcut. Remember you definition of half-life...the amount of time it takes for something to be reduced by half. So 100g would go to 50g then to 25g then to 12.5g and then to 6.25g which would give you 4 half-lives and therefore 40 years.