Suppose a quantity Q(t), with t measured in years, decreases by 4% per year.

(a) How long will it take for the quantity to decrease down to two-thirds?
(b) How long will it take for the quantity to decrease by a half?
(c) What is the continuous decay rate of the quantity Q(t)?

After 1 year, you end up .96* original amount.
After 2 years, you end up .96*.96* original amount.

So after n years you have .96^n * original amount.
Therefore, .96^n = .6666... is the equation to solve for a).

Use logs: n*log(.96= log(.6666) approximately. You can finish this.